No, airlines won't keep an empty seat next to a minor. That would be crazy - a big loss in revenue. Anyone can be seated next to a minor child. I'm not sure why this is a concern, especially if Mom will be right there.

Look for an airline with a middle section with enough seats together. If you can only fly a 3 seat row, place one child on the aisle seat across from the rest of the family. Expensive option would be to buy an extra seat to be kept empty.