The platform is announced in your booking application in advance. You can wait anywhere and if it isn't shown a few minutes before departure, you can go near the platform and see where it is on the screens.
I'm French, I don't have a car so frequently travel by train and I've never waited in the station. Families aren't an issue for me either and it's easy to overtake people if you know where you're going. Especially in Gare de Lyon:
- https://www.ratp.fr/sites/default/files/2019-08/gare-de-lyon...