The zoo is expensive, so if you're on a tight budget I'd be aware.
We went to the underground hospital which was really interesting, not sure how much it'll grip a 2.5 year old, the 6 year old might find the tunnels cool though...
Gorey castle is good to explore, and it's a really pretty harbour.
If you get a chance go to the Old Court House in St Aubin, it's what used to Diamond Lil's place in Bergerac if you're old enough to remember!!! Great atmos there, nice food too.
If they're up to it, Queen's Valley reservoir is a nice walk, it's not too long and it's a nice place for a picnic.
Would second Plemont beach, there's a fantastic cafe at the top of the cliffs, by the steps down to the beach.
Talking of food, you can't beat Big Vern's for breakfast, it's on St Ouen's beach (itself a great beach, goes on for miles and has fantastic sand dunes)
Would second St Brelades as great for a day on the beach... really lovely beach, rightfully the most popular on the island.
The cliffs on the north of the island are great for walking but only if you're happy with your 2.5 year old walking around there as they are high cliffs and there aren't too many handrails at the edge.
Corbiere lighthouse is also worth a look, craggy shoreline, picture post card lighthouse... free !!
There are lots of Martello towers and gun towers left over from the war dotted around the island (the Martello towers are actually Napoleonic but were used by the Germans too) which the children might like to scramble around in.
Dh is from Jersey, so we've been there a good few times!! Can't wait til ds is a bit older to appreciate where his dad's from!