Mine are 10 and 6, and we bought an xbox one for Xmas primarily for the older one, but the younger uses it too (a bit, just for roblox). Bought the ultimate game pass (for the next 2.5 years, thanks to the upgrade deal they offer on gold), but even the eldest spends most of his time on Roblox, which is free anyway
Eldest has two friends on xbox, only one on Switch, none on PS. Others only game on tablet or, one lad, on laptop. If lots of friends are on the same platform, best to join that imo, regardless of increasing cross-play
Previously had, still have, a PC set up to the living room TV with xbox controllers. Just the older one played that, from nearly-8 when I set it up. Games like Minecraft, Trine, Sonic Racing, Gauntlet, FTL, Injustice, Costume Quest.