@beverend I found one thing that really helped was having a SUPER CUTE modern diaper, so they can see, touch, and feel how much better they are.
Other than that, just stick to your guns. "It's something we're interested in, and are going to try." "We are interested in both the environmental and financial side of cloth diapers." "Yes, I might change my mind, but we're going to try it, and I'd appreciate if you'd respect our decision."
Though if they are anything like my MIL, who LOVES to argue, and will argue about anything, just to argue, just shut it down. "Yup, we're cloth diapering" and then don't engage any further, and just shrug and answer with one word answers until they get bored and drop it. Luckily for me, she didn't argue about cloth diapers, so I don't have any specifics for that one, but FUCKS SAKE she was on our case for MONTHS that we didn't do a daily bath. Got to the point I'd just leave the room when she brought it up (luckily it was mostly on the phone with my husband, so I'd just leave.)