Hi,
April is nice. You can expect nice warm weather (like in the upper 80's & 90's) and humid. Being from NY state, you will probably feel just fine in shorts for the vast majority of the time. You can also check out averages for Orlando on weather.com.I would suggest spf 35 or above for your kids. The higher the better and reapply during the day. We are fair skinned and like to use Coppertone sport because it sprays on, it's non-greasy, and it has worked very well for us. I use spf 45 on ds since he's a baby. I just bought the infant banana boat sunscreen for him. Last year I used waterbabies but he didn't like it so I'm trying something different this time.
The only water ride I even give consideration to when picking my outfit for the day out is Kali River Rapids at AK. You can get extremely wet on that ride but with the heat it's usually not a problem. Just don't wear heavy fabric like denim or you'll regret it! One nice thing WDW did was install some heavy duty high pressure air dryers in the bathrooms right by that ride so if you're having a wardrobe emergency you can dry off there. But of course if you feel better bringing some spare clothes then that is ok too. I just never wanted to carry them around so I try to dress appropriately and not worry if I get wet if it's warm, or wear a poncho on the ride if it's not so warm. Splash Mtn @ MK can get you wet too, but I have never gotten nearly as wet on SM as I have on Kali. If I don't feel like getting wet that day, I just wear a poncho.
hth and have a great trip!