Is it better for children to buy pajamas in summer with long sleeves or short sleeves?

It depends on whether the air conditioner is turned on when sleeping at home. If you sleep with the air conditioner on, you'd better buy long-sleeved ones.

If there is no air conditioning, only a fan, buy short-sleeved pajamas. Because of the heat, it's just that the fan has gone down. Of course short sleeves are better and cooler. On the contrary, long sleeves will be much hotter, and children will be uncomfortable wearing long sleeves.