If your kids are the type to fight over something different, then buy the .243. No doubt it will make sight in and ammo a bit easier. However if they are just glad dad has supplied them a gun and happy with going hunting whatever one you should dole out, then buy the 7mm-08. Here is why I say this, when they reach that age of needing their very own cannon, you will most likely be left with their starter rifles. This option you'll end up with a .243 and 7mm-08 vs. 2 -.243's. The oldest or most experienced should have first crack at the 7mm08 and the newest just likes their sibling will start off with the .243.

Best of luck!