Depends what is running, some rides may not be (so never promise children they will definitely do a particular ride).
I agree with polarpercy, fairy tale brook is a ride to do. Duplo Train is great as they can go on their own, or you could sit with 3yr old and 6yr old may get to go in the front and pretend to drive!
Are you going alone with them? If so then the biggest issue is that some rides won't let you take two children at the same time - think things like rollercoasters where an adult needs to sit next to a child.
Boating School (Coastguard HQ I think they have called it this year) will sit 3 in a boat. Go to that quickly, as it tends to get long queues if park is busy.
Atlantis - watch fish from inside a submarine ! Ride does not stop, so prepare you children for boarding it whilst it is slowly moving.
Spining Spider
Balloon School
Sky Rider
Desert Chase
Puppet Theatre (by Fairy Tale Brook) is good - opportunity for a sit down with a
whilst the children watch the show. 11.30am, 12.30pm Three Little Pigs - 2.30pm, 4.30pm Litle Red Riding Hood.
The stunt show is in the Harbour... 12pm, 2pm, 4pm. Good place for a picnic lunch and another cup of
plus there are toilets nearby.
Damp weather - head to Atlantis, 4D Movie theatre, Imagination Factory, Starwars Mini Land, Olivia's house.. as those are indoors! Also Heartlake City Express (big train) is ok in the rain, you get a bit squirted anyway.
On arrival I find it best to head towards Starwars Miniland and go past it, down the hill, past Viking ride, down to Spinning Spider. Then can go for a play at Castaway Camp.