Kylie pads are what you use for elderly incontinent people. They come in a variety of absorbencies, middle one is about £40. For a child you could go for the cheaper one, least absorbent i would imagine. Expensive but they last and do the job well.
SOrry can't remember where we used to get them from, used to order from a catalogue, i'm sure if you do a search for equipment for incontincey something will come up.