If you plan on utilizing family boarding you don't need to worry about checking 24 hours prior to departure as your boarding position doesn't matter. Now if you have multiple adults in your group it would be a good idea to check-in as you'll be limited as to who can use family boarding. If you purchased a ticket for your child, and it's not a child ticket, you actually don't need any identification or proof of age for the child. If you purchased a child ticket (which is just a discounted Anytime ticket) then you'll need documentation proving the child's age. Lap children (under 2 years old) also require age verification, but that doesn't seem to apply in your case.
When boarding begins with the A group stand near the gate agent, but out of the way of the boarding passengers, then after the A group completes boarding the agent will call for family boarding.
Enjoy your trip !
--TheMiddleSeat
