The children are eight and nine, no need to infantalise them. Being a bit frightened of a new experience is normal, and it is good the mum is giving them the space and experiences to grow up independently.
For a lot of children, independent travel overseas is a necessity and perhaps the mum was preparing them for their next trip alone?
Also don't see why mum should not give herself more luxuries. She has earned them as an adult. If she drives the latest BMW, does she have to buy the same for her child when they learn to drive?
Also, it doesn't sound like the two children were much trouble, and eight and nine are plenty old enough to know manners. And since OP doesn't say her dh had a bad time with them, presumably mum has brought them up well, and understands her children's capabilities.
I fly very frequently, at least a few times a month, and have rarely experienced any problems. And there are often very young children traveling alone. Yes, there could be crash, but what is the likelihood? You can't live life based on the worst case scenario. And in any case, this mum was close enough to sort things out, if needed.
It is fine if you do things differently, but why judge this mum? You wouldn't like others criticising your parenting decisions, so why not give this mum the same courtesy. Sounds a bit like class envy to me.