I’ve tried to find out from this forum in the past how this was handled back in the day. I guess most people my grandmother’s age don’t go on the Internet or those who were alive back then were too young to remember, but I ended up asking my grandmother.
She told me that people didn’t bring young children to mass back then. She never did (she had six) and never remembered anyone else bringing them to her parish. My grandfather would watch the younger children while she brought the older children to mass. He was a Lutheran, so it worked out well in their situation (although he was in the process of joining RCIA the year he passed away). But other families where both parents were Catholic had to take turns going to mass. When they had their first child, there were two priests at that particular parish. The priest who was not saying the mass would actually watch their son, pushing him in the pram outside or in the sacristy while mass was going on. Again, people didn’t bring babies to mass at that church either.
I don’t know if this was the case everywhere, but it was at all of the parishes my grandmother was part of growing up and as an adult. We’re from a very Catholic area, too, where it wasn’t strange to see three Catholic parishes within a couple blocks of each other.