Depending on what time, day and what route you take, it could take anywhere from just under an hour to probably 2 hours from DL to MM. I live about 3 miles from MM
We went to DL Monday and it took us just under an hour to get down there (left Valencia at 9:15, took the 5 to 170 to 101 to 5). It took us just over an hour to get home (left DL at 8:45).
There are plenty of rides for school age kids IF they really are roller coaster buffs. My girls are afraid of Go Go Gadget, so MM is only a 1/2 day park for them. But if you're kids enjoy and are big enough for rides like Screamin, they should be good at MM. Your 5YO should be tall enough to ride all the 'fair' type rides (probably 5-6 spread throughout the park), the water rides (2 flumes, a raft, a boat, and a slide), and Ninja. I would think 8YO would be tall enough for everything else up to the newer 'thrill rides'. So the old schools like Revolution, Colossus, Gold Rush. 5YO is probably also small enough to ride the 'kiddie' rides, but they're VERY 'kiddie' rides (just right for my DDs! LOL).
As far as the crowds. Definatly different then DL. The park itself is not as clean either. People you would consider 'rude' at DL or your local mall will be the norm at MM. It's more like a 'fair' type crowd. Your not going to get shot or anything though, just don't expect a white middle class suburbia crowd cause that's not what you'll be getting