Technically, what you will be doing isn't going to be "socialization" at all. In behaviorist terms that is something that can only be done during the dog's socialization phase. Some might consider this just a matter of semantics, but words have definitions for a reason and using the correct word can help us devise a plan of action for our dogs. The OP's question has been asked enough times by others that I think it warrants a thorough, detailed response about all the methods we can use to effect how our dogs react to stuff they encounter.
SOCIALIZATION takes place during the developmental period of 3 to about 12-16 weeks of age. (Really it starts at birth but since the pups aren't very mobile and the eyes and ears aren't even open then, it what goes on in the early days has more to do with sensory stimulation and the mother-pup-littermates bond than with anything else.) This is the time when, if you were to compare them, the puppy's level of curiosity and willingness to explore things is at its highest while the level of fear and apprehension to things is at its lowest. It is the perfect time for the puppy to experience lots of people, places, objects and activities because it is during this period that the puppy is developing its ideas of what is "normal stuff" in its world. If the pup never encounters a certain type of people or thing during this time, the odds are extremely high that when the pup later encounters that thing that it will classify the thing as strange and unusual. To an animal, "strange and unusual" stuff is usually stuff that should be avoided, or chased off. This is why it is so critical that a pup experience a lot of stuff during this time and that it ALWAYS be in a really fun and positive way. Make sure your pup is exposed to kids--lots of them--but not necessarily all at once. Don't force the kids on the pup or the pup on the kids. Just let them interact in ways that teach the pup that humans come in small sizes, and these mini humans make strange noises, smell funny, and move erratically. Interactions should be at whatever level maintains a fun, relaxed and positive experience for the pup. For some pups that is just watching kids play, for some that is getting a treat from and being pet by a kid, and for others it might be snuggling in the lap of child or playing ball with a child.
If you have an older pup or an adult dog who might not have had the benefit of good and proper socialization, don't despair. While you might have a harder and longer road than if the dog had a stronger foundation, there are plenty of things you can do to help your dog navigate its world comfortably and confidently. Depending on how a dog reacts to certain things, a handler's approach may include just one or many methods.
Remember Pavlov? If a dog just has a neutral response or no response to a stimuli you can classically CONDITION a positive response by pairing that stimuli with things that make the dog happy. This is how dogs wind up loving the sight and sound of their food bowl. Initially their food bowl means nothing to a puppy but when the sight and sound of it is paired with food over and over again, eventually just the sight and sound alone elicit a happy, excited response. (You can condition a negative response to a food bowl just as easily if you were to instead throw it at the puppy.) So, if a dog didn't really have any response to children, you could pair the sight and sound of children with good things for the dog and the dog would eventually enjoy and look forward to the sight and sound of children.
Say a dog already has a strong negative emotional and physiological reaction in response to some stimuli-- again let's use the sight of children as an example. A good handler would DESENSITIZE the dog to children by exposing that dog to gradually increasing intensities of versions of children starting with the least intense version so that the dog no longer experiences the negative emotional reaction to children. This might mean that the handler sits with the dog at the park day after day watching children play. First they sit 50 feet away, then 49 feet away, then 48 feet away, etc. Or it could mean that they sit 20 feet away from children playing calmly in a sandbox, then they sit 20 feet away from children walking around, then they sit 20 feet away from children running around, then 20 feet away from children running and yelling, then 20 feet from children running, yelling, and waving their arms. It might mean they sit with the dog near 1 child also sitting, then 2 children, then 3 children, etc. It is a process that deliberately exposes a dog to increasing levels of the stimuli to lessen the dog's reaction to that stimuli.
HABITUATION is really similar to desensitization in that it also lessens the dog's reaction to a stimuli. In the case of habituation though, it is a natural process whereby repeated exposure to something causes the dog to learn that the stimuli is meaningless. This usually happens with stuff in the dog's environment that doesn't change much or increase in intensity. An example would be a dog learning that the sound of the air conditioner cycling on and off is meaningless background noise. Eventually the dog doesn't even notice it anymore. A dog can habituate to sounds quite easily. Expectant parents can buy CDs with sounds of babies babbling, squawking, cooing, and crying to play over and over again so the dog learns that these sounds are meaningless and to ignore them.
A program of COUNTERCONDITIONING is usually used along with desensitization in dogs who exhibit and unwanted response to stimuli. This refers to the process of replacing the dogs response to a stimuli with a different response to the same stimuli. This would be the case if you changed a dogs fearful response to children into a happy response to children. You do this by pairing the exposure to the children with things that elicit the new (wanted) response. Every time a child comes into view you feed a dog wonderful bits of chicken. Provided the chicken makes the dog happy, over time the sight of children will make the dog just as happy as the chicken does. Think of it as "classical conditioning done to counter an emotional response". So now the handler is sitting with the dog 50 feet from children at play (desensitizing) while they are feeding wonderful chicken bits to the dog (counterconditioning).
So decide what your dogs response is to "X" and then pick the method or methods to change and improve that response.
For the OP, some things that you might want to include in your program might be:
Sounds of babies and children
Sounds and sights of baby equipment (strollers, baby swings, rolling toys, etc.)
Crawling children
People/kids with staggering, erratic and unusual gaits
Being handled all over (for if the baby ever accidentally is allowed too close to the dog)
Things being dropped nearby (kids are forever dropping things)
People walking by while the dog is eating
Learning to trade valued objects for treats
