I agree. In fact, as a DM flying in BE because I paid for it, I would complain strongly to the FAs if I saw the OP in BE after the door closed and even if the OP and the kid sitting with him were in the BE cabin for very long during the boarding process. I would then call for the purser and if the OP or his other kid continued to visit BE, I would complain strongly to DL after the flight, with crew names and details.

After the medical emergency in which the OP assisted, he alone could have been moved up to BE at the purser's discretion, but that would require leaving the other kid alone back in coach, which is not allowed because the UM fee was not paid (and the kid might be too young to travel as UM anyway).

Having one or two seats in BE does not permit someone to visit BE or be moved up even if there are empty seats, especially with small children (who cannot travel in BE with nonrev parents). If I were on that flight, I would not accept having the OP's family situation disturb my experience in BE.

This is the first time I've ever been tempted to make the common FT refrain "if you want to travel in BE, you should purchase a seat in BE."