As reported in the internet (!) Yated:
The gemora (Brochos 28b) teaches us: "Prevent your children from [engaging in] higoyon." Rashi explains this saying of Chazal to mean: "You should not accustom them to study Chumash too much since it appeals to them."Yes, the specific context is to forbid computer games (not a fan myself). But once we are explaining that Berachot 28b means that a) children shouldn't study too much Chumash and if so 2) kal ve-chomer they shouldn't play "valueless games", at what point does this not mean "fun is forbidden"?
If Chazal instruct us to deter children from studying Hashem's Torah in an easy and appealing way since it prevents them from laboring over Torah study, surely they forbid children's playing various sorts of valueless games that cause them to detach their thoughts from Torah study. Playing such games cause a tremendous decline in the child's level of spirituality.
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