+1 on buying the extra seat and using a car seat. The FAA doesn't require it, but it is recommended. If there were ever some sort of explosive decompression like happened on the Alaska flight this week, you'd be glad your kid was and/or could be strapped into a seat and not merely held in your arms. Same in extreme turbulence. If it's at all possible for you, buy the seat.

Second, in addition to not being in bulkheads, you can't put the car seat between another passenger and the aisle, unless they're in your travel party. So, you can't buy aisle/middle and leave some stranger to book the window.

Third, you also can't book the exit row (which seems obvious), but I've also had a flight attendant tell me you can't book the rows immediately forward or aft of the exit row either. At one point I found that in writing on Delta's website, but it seemed to disappear years ago. And I've had FAs more recently tell me that's no longer in their manual. Which could explain why I've only once had a flight attendant make an issue of it. But be aware.

My youngest is six now, so I don't deal with this stuff much any more. But traveling with kids of any age is always an adventure. Pack toys, any snacks that are age appropriate, and lots of extra time.