My kids are in and out of parents-in-law’s house all the time. The eldest goes alone on their bike.
Last year brother-in-law and sister-in-law split up and I started a thread about it on here.
B-I-L now lives with P-I-L and his kids visit him there but also takes them for pizza etc. My M-I-L was a bit upset as she feels his kids aren’t happy in the house. B-I-L’s youngest is same age as my eldest and are the same sex. They don’t fight or anything but in spite of my kids’ efforts the cousins show no interest.
Now ExS-I-L is saying that she doesn’t want my kids in my P-I-L’s house when her kids visit their dad and grandparents.