3.5 yo DD has chicken pox. DH went out to get Piriton this morning but he came back with one for 6 yr+. I went out later but pharmacy was closed and best Tesco could do was Clarityn for 2 yr+ but when I got home and read instructions it turns out is only fine if your 2 yo is over 4.5 stone 
DD is 2 stone. Can I give her half a Clarityn or quarter Piriton?