Hi. Your kids are the perfect age for
Legoland! Our son just turned 6 and LL is his second favorite amusement park in SoCal (after DL of course!) One full day will be enough for the main park. As for schools:
San Diego - not in session, though the cities of Oceanside and Carlsbad (where LL is located) ARE in
LA - in session
Orange County - not uniform amongst the different cities, but more are in session than not, Anaheim IS in
So if the choice for your Sunday and Monday is DL one day, LL the other - I personally would definitely do LL on the Sunday, DL on the Monday. The Sunday will be very busy at LL. And if it's hot (and chances are it will be) the waterpark at LL will be crowded. But the main LL park will be nothing compared to how busy DL will be. Just get to LL at opening to get in some rides without long waits, then use your afternoon for the more interactive
lego activities/play areas.
The Monday at DL will be soooo much better than a Sunday since LA and so many of the OC schools will in session on Monday. We're in San Diego and love spending a couple days at DL the last week of August - very nice crowd levels.
**Now if you could do LL on Monday, DL on Tues, that would be truly ideal. And if your kids like water parks, you might consider doing the LL water park on Sunday (just get there at opening so you get chairs and nice spot to relax between slides and all.) DL to LL is only about 45 min. - an hour driving. The LL water park is great for your kids' ages and very clean and they're opening a new section next month. LL has an awesome multi-day parkhopper deal online.