Hi op I'd avoid Crealy it's hellish expensive and hideous(there was a free week offer on though but that may have ended, definitely wouldn't pay full price for 1 day).
The Miniature Pony Centre is gorgeous and one of the cheaper attractions round here.
Teignmouth is nice super play park,nice beach,my lot love riding their bikes along the sea front. Free
You can go crabbing off Shaldon beach opposite and it has a smugglers tunnel to walk through, you find loads of sea glass there.
Lustleigh on Dartmoor is a beautiful village with a little play park in an orchard with a stream.Free
Loads of child friendly walks and streams on Dartmoor.Free
Bovey Tracey has a nice playpark and dis used railway track you can walk to Lustleigh along. It has a nice craft centre with lovely cafe that has toys in for the kids to play with.
Ullacombe farm shop has just opened a lovely new cafe with toys for the kids. There are a couple of animals to look at.
Occambe farm near Paignton is fab loads to see and do all free but if you pay 50p deposit they'll give you a stamp pad and you can letterbox along one of the trails. It has a play area and cafe etc.
Stover park is free. It has a lovely Ted Hughes poetry trail,ducks to feed,education centre,plenty of places to romp around,plenty of trails.-fab for bikes.
As Littlelamb said Exeter is fab-the Apple store is good too as they let the kids play on the computers.The cathedral and library are good too,Boston Tea Party sells the best coffee and is child friendly.Big Waterstones with good kids section. Would second the other Exeter recommendations.
My favourite beach ever is Blackpool Sands free but it has the lovely Venus cafe which can tempt. It's beautiful though and it has a secret garden to visit behind,not pricey.
Bigbury is my 2nd fav and you can go across to the island on the sea tractor.
Paignton Zoo is pricey but worth it,has an under 5's area that is good like a mini pre-school you just turn up.
I could go on but suspect my post may well be a tad tedious now. The key thing is to picnic everybody does down here as we're all broke. I wouldn't spend anything like what you're planning to spend and I have 3 dc. Last Easter hols I think the dc and I only spent £30.
Have fun