Good point, but it negates the JW viewpoint that by letting the child die they feel that all of them will be reunited in paradise. Their thoughts aren't about what's good for the kid today. Their doctrine robs them of normal human feelings and replace them with fantasies of playing with pandas together if they forego this ultimately worthless life.
So if you want to ask those questions you probably first have to defuse the bomb that is their belief system that drops on their child. It's hard to use normal human reasoning with abnormal fantasy.