Good luck to Zia on her surgery.
She looks like such a grown up girl on her Avatar now.
I used the Onesie on Cody . I bought him an X-Large Toddler/short sleeve "Gerber brand" - it says for 35 to 42 pounds,( he was not that weight at 6 months old) but the weight is not the issue, so much as how "long" the onesie is when snapped - otherwise if it is too short, it will pull around the neck when you try to snap the crotch. If you get one, take it out of the package and you will see what I mean. Cody was a longer/bigger boy though. So go by the length of the shirt when snapped.
It did work for him, although one morning, I found him in his crate and the onesie was off - he must of been very busy during the night.
T-Gal,
Love Emma in her purple - don't the ladies in the Red Hat Society also wear purple - I think you and Emma need to join