Yabu A bit
I don't understand 100% child free weddings, to the extent that nearest and dearest can't attend but Children, not babies, take up places and cost money. Children of immediate family sounds like a reasonable compromise.
Where we got married there was only space for 100 people for the service and main meal. Children had to be included in the numbers, despite being small, for fire regulation blah blah blah . My immediate family is large and my cousins have many children! The policy at my wedding ended up being nieces and nephews were invited. Then out of our close friends there was only one child, so they were invited. That was my long time bestest friend, no way would the entire family unit have been excluded. Luckily my dp had no friends with children, or we'd have had massive number conflicts.
No siblings of cousins were included but we did have a contingency for people who couldn't make it if they couldn't bring a child, we had 2 such requests.
It's not an insult that they haven't invited your child, it's probably a numbers game. If you invite one lot of siblings kids, the you have to invite them all. How many places does that leave? Then if you invite kids from one friend, then you have to apply that same rule..