We don't know the diagnosis of the child, the stage of the disease process, or whether the child is on chemotherapy now or in the past nor what chemotherapy was given. Thus, we don't know the immune status of the child and whether his/her body can even respond appropriately to the vaccine. We also don't know whether this child has risk factors to taking the vaccine in the first place which is far more common in these patients (those complications are also magnified in these delicate cases just as the effects of COVID is magnified). We don't know what the advice of this child's doctors were to the parents. It is quite possible that in this case, the risk of the vaccine is high enough compared to the limited/if any benefit that the parents are doing the smart thing. On the other hand, those things may not be present. We just don't know enough.
I would absolutely say though to those who condemn these parents as plague rats when they don't know these other critical pieces of information, that their ignorance is exceeded only by their hate. Conversely, Ronald McDonald Houses do the work that noone else does. It is absolutely wrong to condemn them either without knowing more.