Where can I find a bracha for Ben/Bat Zug for my child? I can’t seem to find anything like that in the regular siddur.

In the regular Siddur you wouldn’t find one but don't bother, make your own Tfilah, Hashem likes you for you are his daughter; talk to him like to a good father, it can be in simple words, in your language. In addition you can always say Tehilim or study Torah in merit of your child. It should be his/her business as well to pray for himself.