I know you already bought a brushless system. I personally would have went with the VXL system as well. All the other systems mentioned are good systems. But your not trying to race or be competitive with your trucks. So the VXL would have been a perfect system for you. First of all its waterproof, Second its programmable without plugging it into a computer or using a field card. Just a simple push of the power button the ESC itself. Plus you could have put it on 50% power until you kids got use to the lipo power. Third its a plug and play system seems how its an actual Traxxas system.
Nimh battery's will work just fine with absolutely no issues on any brushless system. Its just some of those systems you would need the castle link or field programming card for other systems to change it over to a Nimh battery setup. With the VXL you wouldn't need any other extra devices to make that happen. And you don't necessarily need lipos just to achieve longer run times. A Nimh battery with a high Mah rating will give you the longer run times.