Hi OP, I'm lucky enough to have been a few times and as other posters have said I would definitely go for October rather than Easter as Easter is so busy. Having just come back from Christmas there (which is also super busy) it makes such a difference for it even just to be normal busy! And if your kids love Halloween it will be great for that, you could even see if you can get a date which aligns to the not so scary Halloween party.
Also worth a look are websites like Thrill data where you can see trends in busy ness
Also agree that there isn't an ideal age for Disney, but if there was 11 and 7 are both great. We've just been with 9 and 7 year olds and they were big enough to go on everything in the parks (although they were things they didn't want to try!). Also my usual pretty lazy walk refusing kids seem to keep their stamina fine when faced with Disney-we were doing over 20k steps a day without moaning!
I would strongly recommend staying in Disney accommodation if your budget can stretch to it. Particularly as you mention having days you spend time in pools too. Our typical model is to go in for a few hours til lunch, come back, chill by pool and back in late pm. You can use free transport from
Hotels for this (saves extortionate parking charges in parks).
Staying onsite also gets you into the parks 1/2 hour earlier when it is quieter reducing the need for genie/fastpasses which cost more. While it may seem early to get up on holiday because of the time difference your body clock will start off closer to it anyway. It also gets you free parking and benefits like being able to book restaurants for your whole trip for the 60 day window. It's not the cheapest option, and if you can't stay onsite I would consider budgeting extra for fastpasses and parking.
Also do think about whether universal is worth the cost to you if your kids aren't into thrill rides. If they aren't big fans of Harry Potter/other universal theming I'd consider Disney only with then day trips to places like nasa. They are enough to fill your time if you're including the Disney water parks too and pool days. Of course if they are big fans of universal that's fine it's just a lot of money if they won't go on the bigger stuff.
Whatever you do good luck and enjoy!!!!