Sheikh Ahmad Kutty, a senior lecturer and Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states the following:

Children passing in front of a praying person do not in any way disturb Prayer. We read in the authentic hadiths that the Prophet’s grandchildren used to climb on him while he was prostrating in Prayer; yet he patiently waited for them to finish before rising.
One should put a prayer rug or something to indicate their Prayer space, so that they may not intrude. It is OK for them to pass beyond the Prayer space.