2 kids bikes - only small bike left now,

only small bike left, big bike taken - small one would fit a 6/7 years old slight scuff on seat brakes need to be fixed via a service. i dont know how to fix , apologies no longer available - taken - bigger bike - fit a 9/10 year old brakes need to be fixed via a service. i dont know how to fix , apologies On the big bike,looks like higher gears not working. As in moving between bigger dogs. Lower ones work fine. Im sure easily fixed if you know bikes. I dont