r/essential

So, my boy is just about to turn twelve. He has a laptop that his older sister gave him but no phone. I have two old essentials sitting in my desk drawer and I thought it might be a good idea to update one and give it to him as his first phone. BUT.... I have absolutely NO idea how to do it. So I thought I'd come here to search for wisdom...

1). Do you think it is a good idea?

2). How to go about putting a new custom ROM on it.

Thanks,