I think that you should tell your nosey family members to butt out. It's none of your business where they take their kids. The only reason you may have to be concerned is the epilepsy, but he's 5 he's going to keep growing until he's 16 years old. So dosages will change all the time. Do you suggest because their son has epilepsy that they never go to Disney? Or at least until his meds are stabilized when he's an adult? That sounds so unnecessary and no fair. As for them never taking a vacation since they've been married for 7 years....i don't understand why this is any of your concern at all. What if they finally have some money and they want to take a family vacation? As for 3 and 5 not being good ages thats just not true. Those are great ages to go and it's totally up to their parents how far they want to push their own kids boundaries.
We are going in November with another family and their soon to be 5 year old is terrified of every inside ride....he doesn't like not knowing. So dumbo, aladdin, and teacup are OK but the inside rides like winnie the pooh and peter pan are no go's. Guess what he LOVES disney world. He won't even go to the shows because he's afraid of them. He loves the atmosphere, the characters, the few rides he can go on, the parades, and the fireworks.
Bottom line is to just leave them alone and tell your family that they are over stepping their boundaries.