We have Roblox (kids age 5 and 8). From the start I made a hard "no Robux" rule, which has worked out well so far. This has had a very nice and totally unintended side effect: the kids self select against games that are only fun if you pay.
Eg my youngest likes the clicking simulators. Many of those are awful "pay to win" schemes but some actually have fun game dynamics (not unlike the Paperclip Simulator html game that's an HN favorite[0]). Those are the ones he returns to.
These are the games that are more creative in nature, the games that are more challenging and so on and so forth. I feel that at least 7 out of 10 times I check on them, they're doing a game that involves creation, economics, collaboration, and often multiple of those.