Ok, trying not to tread on any toes (I love Jersey too!!) But IMO Guernsey is prettier and quieter than Jersey, more quaint! I think that Jersey is too similar to the main-land.
Saying that, there is not alot for older children to do here, no arcades or fun fairs (unless you count the travelling ones!) We do have bowling alley and a carting track, and a small cinema if it's not beach weather.
We have fabulous beaches all around the Island, we are in the North of the Island and generally use the beaches in the north but there are some lovely coves on the South coast.
Most of the restuarants cater well for children, there are a couple of soft play areas, The Strawberry Farm having indoor and outdoor areas to play and great food.
There are no fast food places here (no McDonald's, Pizza Hut, KFC etc.,) but there are some nice places to get take away food in town.
Town is pretty small so don't come here if you want tax free shopping!!! I mean for clothes as we have electrical, music and jewellery shops that are tax free.
Look at guernsey tourist info
It usually works out cheaper if you book flights/ferry crossing and hotels seperately.
If you want any more info feel free to ask away!
A x