I wouldn't use it in the bath for the kids to bath in direct. I would imagine it would irritate their skin? Do you have any eukybear rub or vicks? You could put some on their feet and a little on their pj's collar.
Otherwise what n2L has suggested with regards to steaming or vapour is what I was going to suggest as well.

Would your 3yr old understand if you put vicks on a hanky or tissue and get her to smell it to help her?