Is it illegal if my mother baby sit my son,with b2 visa,in us and how often can she come back and fort?

she is a serbian citizen,last time i tryed to extend her visa and ins say NO,they consider babysiting grand son as a work...how,why..what can i do...then she went back home for a month and tryed to return in us with valid b2-they did not let her com in us by saying that she spent to much time in us wit us and she is planing to emigrate,why she would do that when this was her 3th. tim coming in us...how evr they canceled her visa and sid that she can reappla for the new one ...what can i do,are they gona give her new visa?