K.B.
I know exactly what you are going through. I've got a five year old who is the size of an eight year old. I've gotten some pretty odd looks before and even was told he could not be in there by an employee. I try to use the family restrooms when they are available.
I do have to share a story: We were in Lowe's once and my son had to use the restroom. He was determinded to go into the mens room. I just stood outside the door. Well here is my son screaming for me to come help him, because there was no toilet paper. I was at a complete loss of what to do. I kept telling him I couldn't go in there. Thank fully a male employee checked the bathroom to make sure I could go in there and he stood outside the door to hold off anyone.
That's when I had to sit him down and tell him if he wants to use the men's restroom, he needed to check he has everything he will need, not to talk to anyone and hurry it up. Also I told him if he ever felt like ANYONE was trying to touch him. Yell as loud as he could. I just stand right outside the door and listen. If he's taking longer than what I expected I ask him if he's okay.
Best of luck!