Kids ages 6,3, & 2, is it ok to let run around in just their underwear?

I have 1 boy ( age2) and the girls are 3 & 6 it's over 100 degrees and they are hot the girls have panties and little tank and the boy just underwear. I did t have a problem cause they are so little but my friend came over and said it was gross and left is it really that bad?