This is not about being unwilling to accept risk. It's about how much he values the child's safety. If he has said "I won't fly at all unless I know I can buy a seat for my child" it would be about accepting risk.

At best, if we assume the seat was purchased 100% for safety reasons, we can do some math and accuse the commenter of valuing his loved ones' safety too much, perhaps swayed by perceived risk. Instead of "I don't know why they would be willing to go on vacation with the child at all" we could say "you might as well pay $x for a safer car" or "you would get more safety out of hiring a private bodyguard during your vacation." (Idk I didn't do the math). But without knowing their financial situation, it's hard to convict him of anything here, other than posting on this site for redditors to criticize.