Hi Simon,
When we stayed at the Quality Inn(back in 2003)We used Mears,as there was no shuttles to WDW from this hotel.We had to book them the day before and give a time that we would like to be picked up.When we were ready to leave the park we rang them up and said we were ready +they normally said roughly what time they will be there-usually it was about an hour.This was always with a shuttle-we never had a taxi sent to us.Also you may be sharing a shuttle,because if a hotel say down the road had guests who were leaving the same time as you they would also pick them up.The price back in2003 was $12 each round trip.I don't know how much for a child as this was when we went for our belated honeymoon,but i am pretty sure the price is box standard no matter how old you are.Hope i have been of some help.x.