Where is the serial number located on the Girls' 24" Roadmaster Granite Peak bike?

contact the manufacturer on the net
see if there is a pdf down load on a page relating to the make and model
ask your local agent for that brand of bike to see if one is available

Because they are loose on the post. Tighten post. Contact local bike shop for assistance

Greasing is not usually done on this bike. If your need to grease it, you probably need a better bike. Contact your local bike shop

Gears slip because gear cable has slack in it. Take it to local bike shop for correct repairs

At local bike shop.

any open hand wrench will be able to assist you in this. you may need to use an 8 or 10 mm maybe even a 14.

Wow, really? Just tell her to get on the bike and pedal. No need for a manual...are you kidding me?

Looks like they can be bought brand new for $100. Its low end quality in the 'bicycle world'. You might get $40-60 if its in good shape.

Sounds like the internal spring that indexes the shifter is either broken or missing. I believe these bikes came with twist shifters. Those springs are hard to find nowadays and it's just as inexpensive to replace. Your local bicycle shop should be able to find a suitable replacement shifter for your bicycle.