The page about appropriate clothes mentions the part of the Bible that says women should not wear men's clothes and men should not wear women's clothes. Does this mean that it's not OK for a kid to wear the clothes of the opposite gender, or does it only apply to adults? Is it OK for a girl to be a tomboy and wear clothes that are meant for boys? Is it a sin for a girl to play football, not wear dresses, have short hair, have sleepovers with friends who are boys, etc.? I'm confused.
"A woman shall not wear man's clothing, nor shall a man put on a woman's clothing; for whoever does these things is an abomination to the LORD your God" (Deuteronomy 22:5).
The point of this law is not saying that a man or woman cannot wear a particular kind of clothing, such as pants or a skirt. Rather, that in fashion there is to be a distinction between a woman and man's attire, such that you can tell if you are looking at a man or woman by what they wear. In the days this was written both men and women wore tunics with a robe as an outer garment, but a woman's robe had decorative boarders and a man's robe did not. The difference was not huge, but it remained different.
Hair styles are also to be distinct between the sexes. Males are supposed to have shorter hair than females. "Does not even nature itself teach you that if a man has long hair, it is a dishonor to him, but if a woman has long hair, it is a glory to her? For her hair is given to her for a covering" (I Corinthians 11:14-15). How much different is not specified, but if you saw someone from the back, could you identify them as male or female by their hair style and length? If not, then it would be wrong because it shows no respect for the differences between the two genders.
Rules like this were not limited to just adults. "It is good for a man that he should bear the yoke in his youth" (Lamentations 3:27). Children accustomed to following rules when young will continue to follow those rules into adulthood.
There is nothing wrong with a girl who likes sports and rougher play. The reason girls generally don't play football is because as guys get to their teenage years, they become stronger that most girls. The competition becomes unbalanced and the chance of injury is greater. Some sports, such as wrestling, becomes inappropriate because it requires close physical contact and the probability of inappropriate touching is too high. "Now concerning the things about which you wrote, it is good for a man not to touch a woman" (I Corinthians 7:1).
Sleepovers before boys and girls reach puberty is not a problem, but once the changes begin, having boys and girls sleeping together in the same room is not advisable. There is too much temptation for sin, especially with young men and women who are not yet used to sexual feelings.