Your grandkids will grow like weeds and depth won't be a concern soon. (A little bit longer for the one year old, but still, it won't be long).
While depth *is* a concern, I'd rather have them contained to one shallow area instead of potentially life guarding two areas.
Mark out a 18x18 area somewhere with string and then cut it in half to envision the 2 smaller areas. 'Hop in' both sizes to get a feel for the traditional pool vs the sport pool.