[AC Question]

I have a google account synced between my desktop, galaxy s5 and laptop. My grandson has an android phone also, but he is too young to have his own google account. I would like to download a tracking app from the play store onto his phone.
(I already have it on mine.) However, entry into the store on his phone is a no go because of the lack of an account. Can I sign into my google account on his phone to access the store and do the download and install?

Secondly, if this is doable, how do I prevent him from then accessing the various facets of my account, i.e., gmail, drive, etc? Is it possible to get to the app, install it and then make my google account "disappear" or at least be inaccessible to him?
Thanks