It is different for adults that understand or with a challenged person, but I know when my girls where that age(7,8,9) and they saw boys their age in the restroom with them, it make them very uncomfortable. I am not talking about special needs children but typical boys that the Mom's just did not want them to use the men's restroom. They are taught in school and by parents that you use the restroom of your gender. If a 9 year old boy in school decided to go into the girl's bathroom they would probably call the cops on him now. I am sure that if a 9 year old girls went into the boy's bathroom it would be an issue too. Stall door or no, you are vulnerable in the bathroom. And let's face it, those stalls have gaps and are pretty flimsy. We have these rules as a society for a reason, right or wrong they are what they are. It is not right to just decide that rules do not apply to you(you in general, not you specifically).That is why so many places have family restrooms now to handle these situations. These are the rules that people expect and count on and if someone is uncomfortable with the opposite gender in the restroom with them then their feelings should be respected too. No one person's feeling and comfort should be placed over another's.