Near Hot Springs is Garvan Woodlands Botanical Garden that you might find interesting.
But I do have one idea that is a little outside your two hour drive limit but which your kids will love.
Blanchard Springs Caverns is one of the best caves I have been in and would be worth the three hour drive to get there. There's also a great picnic area a couple of miles down the road from the caverns right on Sylamore Creek. The creek isn't very deep in most places and a great place for kids to frolic and wade in the water. Between the caverns and the picnic area, this is a great way to spend a full day away from Hot Springs and full of activities that the kids will really enjoy. A little cooler there, too.
