There are two interactions with children that have any effect. The important one is to do a family relationship interaction with them whenever it is offered. An icon pops up over them. Usually once a day or just after they grow a stage. Stages are infant, toddler, preteen. Each time you do this, they get a good upbringing trait star. You want them to have a 3 star level gkod upbringing trait, which is a 5th trait. This trait will make them better (progress activities faster, earn more) their whole adult life if you choose them to be playable, and the only way they can get it is to do a family event when they are children. Just interacting when there is no family event going on has no effect. Sims you create as adults never have the 5th trait.
When the icon pops up, you can use any playable sim to interact with the child, it does not need to be a parent. It says it's a 1 hour event, but it goes really fast if you help, literally a few minutes.
The only other thing you can do with children (not babies though) is change their clothes/appearance. There are only a few choices, and you can rotate through them by tapping the change clothing (or maybe it's change appearance) that comes up when you tap on the child. It's pretty meaningless but I sometimes do it as some options make girl preteens look like boys which annoys me, although it's actually realistic as young children are pretty androgynous.
If a child's appearance matters to you, the most choice you can have is before you have them. Adoption lets you choose gender, but their appearance will be random. If you woohoo, you can choose how the parents look before (if both are your playables) or even yours only if you are woohooing with someone else's sim. For example if parents have blue skin and pink hair at time of woohoo, it is very likely the child will also. But the gender will be random. But really, you can change every aspect of appearance, including gender, once you make a sim playable, so it isn't like you're stuck with how the kid is. Unlike real life