I live near Legoland, so frequently visit - though tends to be with little one's.
Firstly, regarding discounted entry, there are vouchers around which can be used. £5 off per person Voucher, max £25 off - can sometimes be located online. Tesco Milk at the moment has Pay for Adult, Get Child Free.
With 2 adults and 2 paying children (under 3's are FREE), best deal would be two Tesco milk coupons.
2 x £29 Adult Day Pass = £58, 2 4-pint bootles of milk, probably around £2.50. Total cost around £60.50
If you can't find the Tesco vouchers, try using the other type of voucher available online, Children cost £46, Adults £58 = £104, with voucher save £20, so Total cost £84.
If you will visit more than twice a season, get an Annual Pass. I get one of these every year - the Restricted Pass is very good value, if you visit often.
Measure your children before you depart. The Legoland website contains details about height restrictions. If under 1.3m, then you will be limited in what rides they can do - though often if an adult accompanies them it's fine, but some rides do have Minimum height regardless of if an adult sits next to them or not.
Subject to height restrictions, I expect children aged 7 and 4 who like fast rides would like:
Dragon Coaster
Jungle Coaster
Dragon's Apprentice - while being short, it goes round twice and is quite fast.
Dino Dipper - Tip: to avoid squashing your child, you sit on the Outside, but alas they then squash you!
Wave Surfer - prepare to get a bit wet!
The above can all be found at the bottom of the park - so head there first - perhaps start with Knights Kingdom, then over to Dino area.
Take your own food/drink if at all possible, failing that, if the children like Pasta/Pizza, try Pasta Patch for lunch (located near Duplo Land), I seem to get more for the money there, no idea why.