Children should not be given more money than is needed.
与えてはならぬ is grammatically correct, but it’s not used these days. I recommend Clozemaster to replace 与えてはならぬ with 与えてはならない or 与えるべきではない.
ならぬ (= shouldn’t) sounds like the speaker is a king or a draconian type of old man from a pre-World War II era. Or, the sentence is not an oral command but is printed in posters and posted across the city during a war time.