I would say losi mini stadium truck.. when the bairn was that age he used to get allsorts of rc junk from argos and toy r us for xmas. In the end i got him a ftx just before he turned 4.
Recently bought him the losi just to mess with in the house and garden. By far the most reliable rc for under £60. Wish id know about them a few years back. Comes with full sized wheel tx as well.
http://www.modelsport.co.uk/losi-mic...products/28897