I've done both - taken my kids out (DS is now 14, DD is 11) and went the week after Easter. Both times it was just me and them - no extra adult, outnumbered by kids yikes. Actually, it wasn't bad. We also stayed off site both times.
In 2000, when they were 9 and 6, I took them out of school. However i didn't do my homework and took them out in Feb during presidents week when all of the northern schools are on winter break. it was very crowded. AK was literally elbow to elbow.
In 2002 we went the week after Easter. This was before EMH. It really wasn't so bad. Our itinerary was very similar to yours - we did Sea World, Universal, MK and MGM. Universal and Sea World were much less crowded. I made sure we got to MGM and MK when the park opened. We really got to do everything we wanted. The temperature was perfect - not too hot.
As far as the fear of losing your kids, i guess this is every parents fear but it could happen whether or not there are crowds. I always laminated a little card with my childs name, home address, hotel, and my cell phone number.
If this is going to be an issue that causes a great deal of anxiety then take them out of school and go when it is less crowded. You are right - at their age they aren't missing much. You are quickly approaching the time when taking them out of school will be much more difficult but IMO you aren't there yet. Also, if you go at a non-peak time it would be cheaper to stay on-site.
BTW - we are going again the week after Easter. YAHOO