I believe DL will do their best to seat kids under 5 with a parent. Once they are 5 and technically eligible for the unaccompanied minor program (they aren't in the program, just would hypothetically be eligible) they might try more or less depending on the agent you get.

Anecdotally a few year ago when I was flying with my wife and then 9 y/o daughter, I had a window seat towards the front, and my wife a window in row 23 and my daughter a middle in row 22. I went back to ask the window in row 22 if she would like to trade with me for the more forward window seat. She declined as her stuff was already in the overhead and there wasn't going to be any room for it further up. No problem. My daughter then HAPPILY took my forward window seat and I took her middle...

As soon as this was done, a couple with an infant (couldn't have been more than 3 months old) sat down in row 21. The look on the woman's face who declined the trade was amazing. Just the epic mistake she made washed over her. Now this baby was perfect the whole flight and I forgot she was even there, but it was still very funny in the moment.