I used this one for my daughter whom is very petite and we had the same issue of her bum turning red with hard plastic ones

Dreambaby Soft Touch Potty Seat, Pink https://smile.amazon.com/dp/B009K8NBM2/ref=cm_sw_r_cp_api_glt_fabc_YJ3GR3B7F1DBR17DTJHH?_encoding=UTF8&psc=1