Sorry, I have no experience with the all-in-one version. Years ago, I did make cloth diapers with Velcro closures and elastic for leg openings and partial waistband (I purchased the diaper covers that were similar to the diaper design). These worked great until each kid was about a year old. Then I found that the cloth diapers and covers just couldn`t contain the wet. I was changing their entire outfit (except socks) every time I changed a diaper. The cloth diapers went from being easy, to generating oodles of laundry and lengthy diaper changes. I gave up and switched to disposables. I am sure that diaper design has improved since I made mine but be aware that they might not work as well as you want.
There is a big comfort difference between cloth and disposables diapers. Cloth demands immediate changes while disposables can be worn the whole day (a slight exaggeration - the kids does not feel uncomfortable in a wet disposable). At 16 months, this girl will be used to her diapers and if she has not been using cloth diapers, she isn`t going to be very happy with a change. On the plus side, she might decide that she wants to be potty trained ASAP.
If it was me, I would consider a diaper service to test out if cloth diapers will work for this family. And secondly, if they decide to continue with cloth diapers, I am thinking it will not be for long because both the changee and changer will want her trained!
OR if this girl has had only disposable diapers then I would continue with them. (I hate disposables so this is a hard recommendation for me to make) Introducing another change when she will be experiencing many changes, doesn`t seem fair.
Wonderful that your daughter and son-in-law are able to help this little girl.