No one can offer a suggestion without a diagnosis. It can be a wide array of things from asthma, congestive heart failure, kennel cough, bronchitis, pneumonia, reverse sneezing to allergies are something they inhaled that you cant see to take out.
A vet must examine them and determine what is wrong before you can attempt to treat it.
Life isn't about waiting for the storm to pass. It is about learning to dance in the rain.