Baby teeth eventually fall out, so the concept of filling a baby tooth that has decayed seems a little odd. You could just pull the tooth, too, right? You could, and in certain circumstances, that could be preferable, but filling the decay is also a viable option in other circumstances. You have to look at how much longer the tooth is likely to be present, as well as the condition of the tooth and how the child is reacting to the entire situation.
How Much Time Before the Tooth Fairy Is Supposed to Visit?
If the decayed tooth is loose or the child is at an age where he or she is already losing baby teeth frequently, filling a small cavity usually doesn't make sense. (There are always exceptions, of course.) However, a very young child who hasn't lost teeth yet and won't for a few years may need a filling.
Letting a decayed tooth stay in the mouth can have secondary effects. If the tooth is painful, the child will just continue to have pain -- and keep in mind that young children may not know how to describe the pain well. So, if the child seems uncomfortable but hasn't said anything about the tooth hurting, that doesn't mean the tooth doesn't hurt. Plus, leaving decay for too long only leads to more decay and the need for a bigger filling if you eventually go that way with treatment.
Filling vs. Pulling
Pulling baby teeth is common, and if the decayed tooth is also causing impaction for adult teeth that are resting right below it, your child's dentist might want to pull those teeth anyway. But if the decayed area is small, there's no impending impaction, and the tooth will probably be around for a while, filling it may be better.
How Is the Child Doing Through All This?
One more issue is how the child handles dental work. Filling technology has improved substantially over the years since you were a child. If you've never had a filling or haven't had one added for a long time, you may have outdated technology in mind. Dental fillings for kids today are simple procedures that don't take long.
That being said, lots of kids still don't like going to the dentist and are still antsy about dental work. The decay has to be taken care of, but you also have to gauge how the child may react to filling the tooth or pulling it. Your dentist likely has dealt with children with similar temperaments and limits before, so talk this over with the dentist to get a better idea of which procedure may be better for your child.