Anyone near Vanasthalipuram who repairs kids' gear cycles properly?

Bought my 9 year old a geared cycle last year. The gears have started slipping and the chain falls off on slopes. The shop near us oiled it, charged me, and it was back to the same thing in two days. Looking for someone around Vanasthalipuram or Hayathnagar who actually knows gears, not just puncture work. He's been riding around the colony every evening and is very upset to be grounded 😊