What Causes Foam Bubbling Out Of The Mouth Of Toddler?

2 1/2 year grand-baby frequently suffers with colds and allergies. Over the last five months she had 2 x-rays done and both revealed that she had a slight case of pneumonia. She was treated with amoxicillin. She is now suffering with a cold again. The doctor said that it was some sort of respiratory virus. She is coughing uncontrollable, you can hear congestion in her chest when she coughs, she also, have a lot of nasal congestion. I observed while she was sleeping some sort of foam bubbling out of her mouth, and all over her lips. What can this be, and the cause? What can I do to help her get well and stay well. She attend Child Care, and there is always a kid sick, they seem to keep passing their colds back and forward. What can I do?