Really how energetic and how good with kids varies a lot dog to dog. And I would consider a lab/terrier to be a medium-sized dog ...

Really visiting the shelter with at least the ten-year-old, then a "trial visit" with the two-year old, to see who appeals to you, would probably be best. So many of the animals featured on http://Dogoftheday.com , http://Catoftheday.com and http://Petoftheday.com start with "Well, I went to the shelter looking for a (fill in the blank, for example I'll use) small black cat, and came home with this big white dog and couldn't be happier" - you never know what will happen until you get there!