They will try, but not guaranteed. From Delta’s website:
”We realize that Basic Economy fares aren’t for everyone – including families traveling with children. While our Airport Customer Service team makes every effort to seat families with young children together regardless of ticket type, customers who need to be seated beside another person in their family are encouraged to select Delta's Main Cabin product that allows for seat selections and ticket changes.”
Or get online exactly at T-24 to try and select seats next to one another.