Tizzy,
Depending on traffic, which back roads you take and the condition of the roads... it takes anywhere from 50 minutes to an 1 & 1/2 hours. Add stops along the way and it could be more time. I prefer to arrive around noonish and then have a late lunch after the river walk. Check the weather and water level before you go. Water level is important with small kids, if it is too high and the current too swift, the kids might not have such a good time. Also, if the water level is too low it might not be as fun.
Enjoy,
VVHT