With notice, 7pm would never have been a problem for my DCs because they were never ready for bed at that time so they'd have been ready to tuck in with us. Back when they were younger they weren't in from nursery before 6, so it was easily 7 by the time we'd got in, and dinner was cooked, delayed by me needing some head space before cooking after 12+ hours on the go. The DCs have never needed to be up before 7:30 so they've always had enough sleep on a later than average shift.
Back to average family patterns, Christmas Dinner at 7pm is an appalling plan for most young families. The children need a decent meal in the day which won't be on offer if the cooking facilities are that poor and the adults are holding on for that day. Many children love roast dinners. I accept that DS1 is an outlier, but Brussels Sprouts are and always have been his favourite food, right since he turned 1 at Christmas 
If the DCs are used to going to bed at 7pm, the experience will be a disaster, because hyped, out-of-routine children are highly unlikely to merkly go to bed bang on 7pm just as dinner is served, especially if they haven't had a proper dinner all day. There will be a lot of parental yo-yoing up and downstairs to settle children, and possibly unsettled children coming down and gate crashing.
Quite frankly a child-free dinner at 7pm is a batshit plan destined to be an unpleasant, stressful fiasco of hyped/ tired children, stressed out parents and a frustrated host.
Even DM got Christmas Dinner served at 3pm (albeit always insisting it would be 1pm
) A few years back she reached her limit on hosting and said it was her final one for cooking herself, and that was fine. If you're not prepared to do it half-decently, don't offer.