I rode a children's bike, now the chain is sagging - is it ok to shorten it?
If this happened over the course of 1 ride, it sounds like the wheel just slipped forward in the dropouts. If you loosen both nuts on the rear wheel axle and pull it backwards evenly, you should be able to retension the chain to proper working specs. Then just tighten it back to a torque of pretty f**ckin tight so it doesn’t slip again!