i bought a trike for dd aged 5 a few months back it was a mission trilogy it was pretty expensive around £400 after adding extras like self leveling pedals and stuff - dd loves it and it gives her a lot of independance even thou she needs me there holding the parent bar.
btw dd has asd, low muscle tone, sld, and really bad coordanation
it took her a good few weeks to get the pedals moving and we have to have her feet strapped in or else there all over the place. but she can really get it moving herself now, just has to master the steering
we've got a quest 88 kitten. (and actually, we're thinking it's probably too small now and having the same dilemma of looking for a bigger one.) dd2 is 6. it's got pelvic and thoracic support, and those pedal shoe thingys that you strap into. it also comes with a removable parent handle, so you can push until she gets the hang of pedalling.
i really like tomcat trikes btw, (we actually ordered one first, but they were having real problems with their suppliers and after several months chasing we cancelled the order - couldn't risk not having it on christmas day!)