If I knew in advance that I cannot fly with my 6 year old seating with me I would have reconsidered the whole vacation plan. I was only offered the option of paying for a seat at the airport (which I did pay for) and NOT at booking time - I wanted a middle seat next to any of the 3 window seats I had assigned - they did not sell those particular seats.
I still think it's not right what Delta is doing - as someone said earlier in a post - not all people flying are nice people or people I would like/feel safe having my children seating by - as far as I know Delta does not run background checks or look for sex offenders when selling tickets or boarding planes. The same way I would not let my children eat at a food court with a stranger I would not want them spending 2+ hours with someone I don't know in a plane. I hope it does not sound unreasonable the argument.

Now I am stuck and stressed because I booked my July trip with Delta before this one.
For houserulz77 that offered some help, thanks, these are the details for the trip: Jul03 ATL-CUN DL531 return CUN-ATL Jul10 DL534 bought the tickets May 3rd. Is there anyway to avoid seat assignment changes or unpleasant surprises at the airport again? Your help is appreciated :-)
We have now windows and middle in row 24 - I like them - the plane was empty when I booked and the seats were opened (exactly as it happened with the Orlando trip).