Heelers especially need not only a lot of exercise, but also a lot of mental stimulation. These dogs are excellent working dogs, and they have been selectively bred for their love of working -- if they don't have it, they will find ways to keep themselves occupied, which often includes chewing and digging things that they shouldn't. I wrote a much more detailed article on their temperament for the blog on Petango.com, and personally they would be one of my last choices for a child's pet. Especially for a 3-year-old, something that's a bit lower-energy and more content to just play and then curl up on someone's feet at the end of the day would probably be best. If the cocker spaniel side is dominant, then this dog might work out -- if not, there could definitely be issues.