Ardor token is your chance to forge a block. The more you have the bigger your chance to collect all childchains' fees in your forged block.
what is the reward for forging a block? more ardor?
Childchain fees.
Correct me if I'm wrong, but if you forge Ardor main chain blockchain block, you get Ardor. If you forge/bundle child chain block, you get child chain native token fees and this child chain forger/bundler needs to pay Ardor to Ardor main chain forgers for child chain block be included in Ardor main chain blockchain.
Correct. It was discussed multiple times but I could not convince people that bundlers and Ardor payments are not needed in this process. 
Often the bundler and the Ardor forger will be the same account. So then there is no separate bundler needed, the Ardor fees don't matter because the forger is paying them to themselves, and the fees they receive are in the child chain tokens. So bundlers and Ardor payments are not actually needed if you do it this way. However, having forgers and bundlers be optionally separate is more flexible and enables some new use-cases.
I'm not sure if this is what you meant. Durerus reply made me think it wasn't.