Under Fair Housing rules, you cannot single out children. However, you can advise a household that a member of their family is behaving in a disruptive way and annoying the neighbor below. The bench mark, I believe, is to address the behavior and not the age of the person causing the disruption.
Here is an excerpt from Project Sentinel, a HUD Approved Counseling Agency:
Child-Related Noise Complaints
Generally, children must be permitted to make a reasonable amount of noise for their age, the activity and the time of day. Babies cry, and kids run and scream when playing outside, and in most cases, they shouldn’t be punished for doing so. Lease violations should only be issued when noise is unreasonable, and if an adult tenant would be issued a violation if he were making a similar amount of noise. Nothing in the Fair Housing laws prohibit a housing provider from having reasonable quiet hours, or requiring tenants to keep music and television levels low.
Here is the link to the page:
housing.org/fair-housing/familial-status-discrimination