I don't think it's sad for the children to travel separately. When mine were younger we didn't have a car and often we ended up splitting up travelling for various reasons.
They found it great fun. Who would get there first, talking about different experiences, did we stop at the same places, did the others see that weird windmill? etc.
And it was often more relaxing for us (although tbf dh almost always took most of the luggage so maybe he found it less relaxing) because we could tailor what we did to the children we had. I remember one particular journey with dd2 where we made an spy story together using the people getting on and off the train (we even got off at one stop and changed transport in case we were followed!) and another one where we took a detour for ds so we could eat in a café by an airport so he could watch planes and dd1 used to love to plan the journeys and decide where to stop beforehand etc.
Now they're pretty much adults, but we were all going on holiday last Easter, and had to take two cars because ds had to get back a day early, and they were remembering about going separately and how much fun they'd found it.
So it isn't something to feel sad for the children. I suspect it's sad for the adults; but perhaps more in a "they aren't little any more. Aw!" sort of way.