My only suggestion is to be careful in that type of situation. The parents for various reasons may not like them participating in cycling with you. Keep the parents informed. Help out how you can and where you can. Enjoy the opportunity to help the kids get some exercise. Don't be upset if their participation ends.
Your doing well with the bike maintenance and going riding with them. The only other possibility is to keep your eye out for bikes they might be able to use. Put out feelers to the parents if the kids keep riding. Let them know you are willing to help them find better bikes for the kids. I would not offer to buy or help buy the bikes, but that is up to you.