There is only one RCI ship out of Southampton this year and that is Independence. However she has been sold out for months for the summer holiday period for 3/4 berth rooms. So you will have to get 2 cabins with everyone paying full price. Best to wait till next year goes on sale around April/May and get in quickly for a booking for summer '09.
There is only one, the Independence sailing from Southampton first class cruise line for any one with kids plenty to do and eat 2yr old may need to be with parents,they will need to book early if going in the school holidays as family rooms do sell out quickly,good choice of places to eat and formal dress is not essential .
Other cruise line to look at is NCL again a brilliant line for familys, the reviews on here give a good insigh.
You may want to check availability on Independence around April - this is when final payment is due and people may drop out as they discover they cannot afford it.
Don't forget the Brilliance of the Seas sails out of Southampton on 19th May with a one way flight back from Barcelona 10 nights later. This ship has a water slide and most of the treats the Independance has with the same high quality kids clubs. If you find the Independance sold out or too pricey and can get the 7 year old out of school you may find this one an alternative of the same high quality (I'll be on it but my kids are grown up now but I have seen your age kids having brilliant times on RC ships - they even sometimes take them to kids evening meals and activities if you want to allow you to have an enjoyable adult evening meal.