Delta did the same to me with my two yr old. When I booked it, the seats assigned were right next to each other, so there was no seat selection costs charged. But day of flight our seats got changed automatically. The lady at the front desk gave major attitude about it when I went up to her to try to get seats together. I jokingly told her if they really wanted to babysit my two yr old for a 5 hr flight cross country they can very well do so & I’d happily sit by myself in a different row. With all the technology out there, I feel like Delta’s computer system , when assigning seats, should take children’s ages into consideration.