Shoes off ok (probably a good idea since kids often sit in their seats with shoes on seats) but only if they're wearing clean socks or slippers. Given the general stickiness of children, I would not want to sit in the seat after a child had been sitting there with his/her grubby bare feet or less than perfectly clean socks on the seat. So - pack an extra pair of thick cotton socks (or slippers, but I would think that socks would take up less space in your totebag) and use them on the plane, changing back to shoes (and regular socks) on arrival.