Thanks for the information. If they wont take the medication as liquid then you may be able to mix it into yoghurt, puddings, applesauce or jam. This will be fine with expectorants and tylenol too.
Not taking medication is a learned response as the textures and tastes are unusual. The key thing is not to let it become an issue. Also give the medication separately if the little one is following big brother/sisters example.
If meds are to become a regular thing then involving the children in the choice of two different flavours can help. Eg if choice is blackcurrant or strawberry, they will be more inclined to accept the flavour they selected.
The use of star systems, and praise for taking all the medicine can be useful and using the psychological pressure on the elder child that they need to help the little one get better by giving the meds will also make them feel more grown up.
Ask your doctor for saline nebules. These liquify the mucus and have the benefit of being able to be used frequently. The children also need to drink plenty of water as being dehydrated will thicken up the mucus. Also some people say that milk can make catarrah worse, so reduce their milk intake for a day or two and see if that helps.
Ensure their bedroom is well hydrated too and a humidifier can be helpful (or a wet towel over the heater) to keep the air moist and help with breathing.
Does this help?