I've been to museums around the world and the ROM is a fantastic place! Their collections are first rate and there's probably more to look at than you have time, interest, or energy to do so.
The dinosaur exhibits were fantastic!
The staff was super friendly and wanted to help explain things.
My 8-yr old and 5-yr old were super engaged and interested --- plenty of kid friendly activities.
They had some Chinese artifacts from the 6th Century!
I'd list this place as a must have in any visit to Toronto. Be sure and get the City Pass if you're doing the CN tower, science center, or other popular attractions. Just the convenience alone of skipping lines is worth it.