If you actually intend to fly with an infant and then cancel at the last minute then there is nothing wrong with this. It happened to friends of mine who were due to come to my wedding. Infant had been ill and they were waiting for test results, which didn't come in time so mother stayed at home with infant. Thankfully the test results were negative so they could have come after all but it wasn't worth the risk.
Of course there is no way of the airline telling if you have set the whole thing up, assuming you have filled in advance passenger info, unless they demand a medical note. I doubt they would though.
Whether it's honest or not is another matter.