i have a 6 year old son with allergies and
Hello, I'm Dr. Branch, thanks for using Ask a Doctor. There are no interactions between the allegra and delsym, so there should not be any problems there. However, we typically don't recommend over the counter medications like delsym for a 6 year old, the evidence has shown that it doesn't help much, and there can be side effects. Instead of this, you can try and we would recommend using some honey, you can use a teaspoon mixed in with juice or tea, and this is a safe way to treat the cough.
I hope that helps, please let me know if you have any other questions and I'd be glad to discuss it further with you.