Buying presents for babies and toddlers is something that seems quite straightforward. And in a way it is, because all children love toys. However, in other ways, it is LITERALLY a MINEFIELD.
What happens when you buy small children toys? They will be very happy for a short period of time and this might make you happy, and that's great if that's what's important to you. But they probably have a million toys already and will be playing with something else very soon, so maybe you can do better. That said, if their parents are very poor, you may want to buy them presents. All children like thinks to do with rescuing people or diggers.
So if their parents are not poor, they probably don't need presents.
Your best bet is to ask the parents what they would like you to buy for their kids.
If you don't want to do this, for some reason, then buy clothes. But they will already have clothes for the age they are now. If they don't you'll know because you'll have noticed them running around naked. Buy clothes for their current size and they'll already be half way to growing out of them. Buy clothes for when they are six months older than they are now.
You could buy books because all parents are sick to death of reading the books they currently have, night after night. The parents will be happy. Well done.
The child will very soon forget what you've bought them. If however you buy a present that makes the parents' lives worse, this will not be quickly forgotten.
DON'T buy something so large that it takes up half of their living room.
Don't buy something really mucky like Play Doh. If they want their kids to have really mucky things, it's best if they do that and not you.
Don't buy anything with batteries. Because it will make a noise and the kids will love it and the parents will remember that you did this to them.