I think 3 days at
Disneyland is perfect. You'll get to ride everything at least once, and your favorites multiple times. But if you want to things slow and easy, sure, you could stretch it out to 4 or 5 days. But I've never done more than 3 and its been just fine. Honestly, I'm not sure kids that age could last 5 days!! I think you'd probably end up spending more time at the hotel swimming or napping, etc, so in the end you may as well just do 3 days and get it over with!
1 day at SeaWorld and
Legoland is fine, too. We've done both in one day and we didn't leave feeling like we missed anything. Again, you could stretch it to two days if you wanted, and I'm sure you would have fun. IIRC, we missed Dolphin Cove b/c the line was SO LONG and only saw 1 of the Sea Lion shows (they have a morning and night one). I don't think we rode both rides either. And we pushed ourselves hard to get through everything else in 1 day. With Legoland, ds9 was ready to go before it even closed. They didn't do the Aquarium though, or the water park.
So IMO, 3 days at Disneyland, 1 day at the beach, 1 day at Legoland, 1 day at Seaworld, 1 day at the beach would be great. If you have more time, or get some extra free at the parks, then sure, stretch it a little longer! But more downtime at the beach might be preferred than being in the parks every day!