My non sn dd still does that at 8!

My 3 year old ASD son puts the sole of his foot up to his cheek. Like you said, not all the time just if he's playing or watching tv every so often.

My kid does this sometimes. He's five with AS. I remember doing the same at his age except I would also trim off my toenails with my teeth. Ewwwww... I know although I'm very good at yoga as an adult.

My daughter went through a phase like this when she was about 5. She would lick her feet, hands and arms. You might want to try chewelry or sensory toys meant to be chewed on or mouthed.

put her foot to her lips?

My 4 year old started doing this a month or so ago. Not often, just every once in a while, she'll be sitting on the floor playing and bring her foot to her mouth.