Can two children, aged seven and four, recite the daily "Tahseen" (invocations for protection) by themselves instead of having it recited for them by placing a hand on their heads?

There is no harm in children reciting the [protective] supplications themselves, and this is preferable so they become accustomed to them. It is not necessary to place a hand on the child's head when reciting the supplications, nor is it necessary for someone who recites them for another person who is unable to recite them themselves.