If you do decide to purchase sandals, make sure that your heel, ankle, and toes are enclosed and protected. Exposed toes on a rocky trail are only asking for a serious foot injury. You are sacrificing ankle support with a sandal and you need to be mindful of that. Finally if you are in an area where you will be exposed to a lot of sun, make sure to apply sunscreen to your feet. Nothing will stop a hiker faster than sandals rubbing on a severe sunburn.
Several companies make excellent hiking shoes for children.
There is no real hard and fast answer to that question. You need to evaluate your budget and your requirements. Expect to pay more for Gore-Tex or backpacking boots. Com has provided this information as an agnostic view on buying boots. You can visit our links library under Gear, Boots to find a listing on a number of boot manufacturers that provide excellent quality boots.