We had our four-year-old in tow. Some easier hikes we did:

* Johnston Canyon
* Louise trail to far end of lake
* Sunshine Village (gondola or bus takes you to the top base area where you can choose from a few different hikes)

For Johnston Canyon/Lake Louise/Moraine Lake - you need to get there really early to be assured of a parking spot. If you've got multiple days, I'd split those into separate days just to ensure parking availability.

What I wish we would have known ahead of time - We continued north along the Icefields Parkway to Bow Lake and Peyto Lake - I wish we would have had more time to spend exploring those and the stuff to the north including Jasper but we ran out of time (maybe another year).