Our first trip to Uni (and WDW) was when DS was 6.5. He was definitely taller than your little one, but he wasn't able to ride everything (nor did he want to ride some things he could have gone on, like The Mummy). And it was all fine. There's so much to look at. And there's a bunch of stuff the little one would probably love to do if you gave him time to do it. (mainly in the Barney area, LOL)
As the oldest of 5, and the tallest for quite awhile (now the shortest), I really balk at the idea of not letting olders do what they are capable of until the youngers can do them. If the majority of the family want to try Universal, then go. As long as the spooky level of the line (lookin' at you, The Mummy) doesn't freak out a younger kid, the family only splits up at the very end. It's not like at Disney where you say goodbye outside of the line. Everyone can go into the lines at Uni. It's pretty awesome.
Woody's Coaster is superfun though also supershort; if you're staying onsite you'll be able to Express Pass it, which helps a LOT with that 30 second coaster. Fievel's playground, a waterplay area themed to Curious George. Follow the footprints in that area to stay dry if you wish, and go back behind to a pretty incredible soft-ball play area. Vacuum up the balls into hoppers, shoot them at family members, they are so soft you won't get hurt. It's really great fun.
If the family wants to go, go. You'll likely be back later on, and the little one will be able to do more and the 8 year old probably will, too, and more fun will be had.