Not worth it, I see it as a gimmick at this time. For one you don't actually own the games with a monthly subscription service. If you buy games digitally on the platform you still don't really own them, you are just paying for a cloud platform to host them (same as a majority of digital content). If the platform goes away in a few years so do your purchases.
Also, digital games on the primary platforms (XBOX, Sony, Nintnedo) typically cost more then what you can find a physical version of the game for. Buying used physical versions of games won't be an option either. For example Red Dead Redemption 2 is coming to Google Stadia...for $60. The game has been out on PS4 and XBOX for a year and you can get a new physical copy for $30 or less.
A cloud gaming platform is also dependent on your internet connection. If your connection speed varies throughout the day, or if theres an outage then no gaming. Even if you have a good internet connection you may not have enough bandwidth if theres multiple other people in the house using the internet at the same time. Suddenly the 4k game is down sampled to 1080p or 720p.
Finally the user community on Google Stadia is going to be barely a fraction of what Xbox and Playstation have. If your kids want to play games with friends that won't be an option.
Personally I would recommend picking up a Playstation 4 pro around Black Friday for $350 or so, maybe a bit more if theres a good bundle.