my experience of buffets is that there is a lot of crisps etc but very little real food. If this is at a meal time, then I would want there to be sandwiches, wraps, sausage rolls etc, not just crisps and cupcakes.
The other thing is that I would never take anything with a dip as kids just bite off the dip and then double dip their carrot or whatever in the dip and yuck.
and last thing is, make it finger food. So if you want to have healthy salad, make it cucumber sticks and cherry tomatoes, not a mixed salad that needs forks.
I think any food is going to suffer from 4 hours in the car, whether it is egg sandwiches or ham. So I would be planning a cool bag, food in tupperware covered in ice blocks, and given that, you can take anything.
mini quiches, pizza squares, decent quiche are all alternatives to sandwiches. Wraps cut into pinwheels make a nice change, but are a fiddle to make.
Sausage rolls, cocktail sausages etc still go down well particularly with a lot of kids and they know what they are.
But (depending on budget) my preferred option would be to get the party trays from supermarket that have mini samosas, spring rolls, onion barjiis etc. They are always the first to go on a bufftet, very popular.