I have a 4 yr old that I am giving Claritin for seasonal allergy symptoms, seems to help a little but she now has a cough this morning. Mucinex (guaifenesin)?

Cough only. I like delsym better because it is for cough only. Or you can check if there is a Mucinex (guaifenesin) that is for cough only. I prefer childrens medications with the least amount of active ingredients to help the symptoms.
Yes. it is ok to use the mucinex--you can use a cool mist humidifier too.