Let your kids help with the test drive: Can they open the doors to get in and out of the back seat easily? Can they operate the controls to get in and out of the third row (if the vehicle has it)?
If you have small children, bring their car seats with you so you can make sure they attach easily and safely. Bring the stroller or other large items you normally carry to ensure it all fits into the trunk. If you're actually planning to buy at that time, be sure to also bring activities and snacks to keep them occupied while you're going through the purchase process.