Hi, good day.
This is not so complicated that it looks, you must first planing your scene, then: in a new synfig document just make a bouncing move in a cycle Loop some like this one:
Then, you can import the Bouncing cycle into the scene that you want make the scaling, just have to add the scaling eff to the imported one, then you can make a little more complicated movements of various characters in the same scene without having to working all they to the same time.
If you want, I post the files .sifz of this animations right here:
You can download it if you want to see how its works.