I usually find help yourself buffets quite time consuming to prepare (even the simplest of spreads) but easy for serving/clearing up etc (esp if you use disposable plates etc)
Think a few well chosen dishes that go together well are better than loads of different clashing smaller plates of things.
And if it's a mix of children and adults then you want to keep it simple.
Think roasting chicken pieces is a good idea
From experience, DC usually love simple cous-cous salads and grated carrot salad and corn on the cob and sausages of course. And agree with person who said cold pizza (you can buy decent ones from supermarket, cook them, and cut them up in to small squares with scissiors - dc usually love them)
Followed by mini-tubs of ice cream (saves ages serving individual portions and usually much less waste) and bowls of strawberries
One of the easiest dishes for parents is a bag or two of prepared rocket salad with a medium rare steak or two sliced up diagonally on top with a good dressing and shaved parmesan on top, lots of s & p
Can be served with tomato salad and/or new pots
Or go for one "theme" such as mezze ie lamb (big roasted shoulder) tabbouleh, hummus, flatbreads, roasted med veg etc etc (could do wraps for children but that's a bit more time consuming)
More ideas here
Lemon or Choc and raspberry tart for afters??