Big Game dougle ladders are not all that expensive and work fine. Suggest you plan on wrapping cammouflage cloth (burlap, etc.) aroung the seat frame to hide some of the movement. As already said, be sure to use a quality fall restraint system.

Ditto .... A large sized ground blind is a great way to go too. Might be better right niow for your youngster .... a bit of protection from the elements, hides his wriggling about (tough for me as a 9 year old to remain still for long !) and there is no fall risk. Double Bull ground blinds are rather pricey but are durable and go up in nothing flat.