When you check the DL Sky Club policy for access to partner lounges, the Skyteam branded lounge at LHR is not mentioned.
http://www.delta.com/traveling_check...ions/index.jsp
at the very bottom it mentioned KL and AF operated lounges and that you can take your spouse and kids. The ST website mentions however just one guest for the LHR lounge.
So while I understand that charging 25 quid for a kid is ridiculous (though it's not just food/drinks they consume, but space & possible inconvenience to others that the fee compensates), writing to Delta for clarification of its Sky Club policy seems advisable.