Saw this yesterday, 5 year old son who really gets into a zone at th movies didn't like the seals and other bits, including the the fear that Mumble may have been lost etc. However he continued watching and enjoyed other bits. 4 Year old daughter was not really into the film and everything went over her head i think!
To be honest I was expecting a cute and happy story, but it was darker than I expected and not as entertaining for the children as I thought it would be.