You should pre-notify your airline, travel agent or tour operator if you use a powered wheelchair or mobility aid. This is because batteries may be hazardous and need special storage. All powered wheelchairs must have their power immobilised before they are stored on the aircraft. This can present the ground handling team with a challenge given the variety of powered chairs. Consider using a simple AirSafe device which immobilises the power and saves handling teams the hassle of working out how to do it. A video is available. Always check with your airline if you are unsure. For more info see here or on Twitter at #aviation #travel #safetyfirst
You can search The British Healthcare Trades Association extensive list to find your wheelchair and see what the airline will need to do before it is loaded.
Most airlines will permit you to take your child in his/her wheelchair to the door of the aircraft. Some will need it to be relinquished at the boarding gate. Very occasionally, wheelchairs may be stored on board in one of the cabins but this depends on the airline, the type of aircraft used on that particular route on that particular flight, the number of passengers on board etc. This is unusual.