That does sound a lot like Usborne. Here's a sample of their cover style from the early '90s.
They've had a ton of science books throughout the years, including generic science dictionaries and encyclopedias. They seem to keep things cartoonish and kid-friendly. I'm still looking for descriptions of their other books of that era.
Eh, crud. Searching 1990s specifically opens up a host of other publishers. Dorling Kindersley, Macmillan, Golden Books and Eyewitness. A lot of possibilities to go through, dang.