I'm traveling with a 6.5 year old, nearly 4 year old, and nearly 11 month old in a week. We've done two trips with two young kids. Here's what works for us:
*We use a single stroller that reclines and has a good sun shield. I use a ring sling to carry baby. The nearly 4 year old will walk or use the stroller. Baby will get the stroller if needed for naps. Draping the sling over the stroller helps get baby a nap! Our oldest will be expected to walk. He's too heavy for our stroller anyway.
*We'll buy some snacks but taking some into the park. Last time, we found that the 6 pack mickey lolypops saved the day. They took a while to eat and were great while waiting ion a longer line.
*Baby swap is awesome. You can swap with your husband - and then the older kids can go twice in a row!
*Measure kids in their parks shoes before you go. Then subtract an inch and treat that as their height. That way, there will be less disappointment. I have one child a tiny smidge under 40" in her shoes so we'll be measuring just for a couple of the best 40" rides but with the attitude of, "you're probably not tall enough but let's check anyway . . . "
*Read about the rides beforehand. Being tall enough doesn't mean your child can handle it. Somethings that sound friendly can be scary. We have an experience with a 3 year old and "It's Tough to Be a Bug," that involved carrying a terrified child out of the attraction three minutes in.
*We do breakfast in our room since it's easier with young kids. Cereal bars, juice boxes, and frsh fruit and we're set. It saves us money and saves us a lot of aggravation!
*Most say to go back to your room for a break mid-day. In the May heat, this may be a great tip. But when we tried it in May/June, it was worse. The 3 year old wouldn't nap and it dragged out our day. We prefer to go at rope drop and stay until the kids act like they need to leave. Tell the kids you're doing this and they may behave better because they want to stay longer! We leave sometime around the dinner hour doing this. It allows me to get the kids to bed at their normal bedtimes.