The second hole is used to prevent the hold from spinning. Typical children won't have enough weight and pull strength to cause well-made, properly-installed holds to spin, but having this hole available gives you the option for higher stability if needed.
I do not recommend buying holds that require two bolts, since you'll need to install T-nuts in the exact locations required for those holds. This will require more work than it's worth -- it's better just to buy quality holds like these, these, or these.
And if you don't expect that you'll want to change the holds or their positions often (or ever), screw-on holds that don't require T-nuts or bolts are much simpler to install; a handful of #6 drywall screws is all you need. The suppliers in the three links I noted earlier also offer screw-on holds.