It looks great with all the right characters and a Fat Controller strolling around, is well themed throughout, has imaginative rides for little ones and is in very good condition (as you'd expect for something that's only been open for a year!).
It does get busy very early, unsurprisingly really and for certain rides the ride itself really doesn't justify the queue time especially with a toddler - on the other hand, some really great rides have quite short wait times. At the other end of the train ride from Thomasland is a good, big adventure playground which also lets them burn off steam and there's a big Thomas themed soft play area next to Thomasland which is in great condition and very imaginative.
The rest of the park is a real mixed bag and very teen orientated - lots of thrill rides - but there are plenty of other things you can do which the little ones like (and can go on), be it splashy water ride or dragon boats. The zoo is an utter disgrace (I actually wrote to the Zoological Society to complain) but sadly, DD did like it - probably because given the bare enclosures you're guaranteed getting a close up of the animals unlike at decent zoos. As others have said, I'd take your own lunch and buy necessities as you go - not cheap, and very average.
I'd check in at Thomasland early, wander around and get on a couple of rides to sate the little one's enthusiasm, catch the Thomas/ Percy up to the top of the park, then amble back through the zoo and the rest of the park, before getting back to Thomasland mid afternoon when it's getting quieter.