I’ve run into the notion that children under a certain age–around 3–should be “lap children” if a Mass is crowded.
My opinion is that once a child can sit and wants to sit by themselves most of the time they should be able to have a seat or if there are pews enough space that they are comfortable.
However, among the older people, my view seems not to be appreciated. Many moons ago at a crowded Mass, someone suggested that my niece (then 7 and not a small girl) sit on my lap the entire time!!! We had arrived early. Someone who arrived after Mass began wanted her seat.