James Eddison wrote:
and if you haven't yet bought your toilet pan, how about going for a
wall-hung one?

Just a thought, but that would allow you a subtle height adjustment...
However, toilet pans are the height there are for very good ergonomic
reasons.
3" higher could be quite uncomfortable for someone with even average leg
lengths, never mind a child.
Its my guess that if you look at the install instructions for any make of
toilet pan they will specify more or less the same outlet height.