We went in February, adding a day at Universal to our two week (mostly) Disney trip. Our kids are 3 and 4 and we decided to leave the kids with my parents, who were traveling with us, for the day. I think it was a good call since we agreed that aside from Seuss landing, there wasn't much they would've liked. We were mainly interested in WWoHP ourselves and since the kids have had no exposure to the books, they probably would've been begging to leave that area long before we were ready. I think if we hadn't had the grandparent option we would've been best off skipping Universal for that age. It's a lot of money for a one day ticket for a preschooler too!
Of course every kid is different. If your child loves superheroes, dinosaurs, comic book characters or "real" roller coasters they'd probably love it. And Seuss landing was pretty awesome for little ones if you do decide to go.