Before getting other people to be involved with Bitcoin, I heavily suggest that you make sure that you actually know what you're doing first. Leaving your funds in Coinbase is a bad idea. Coinbase is an exchange, and only treat it as it is, an EXCHANGE for Bitcoin<->fiat. Not as a wallet for storing your funds.
Read:
That is actually what I was going to address. It seems actually that the OP does not even understand the importance of cold storage when it comes to holding for long term to be thinking of coinbase account for holding which in this case is centralized and in that case, anyone using it does not have any control over their wallets.
Also, it is very imperative to let the niece understand what he is giving her, the importance, what makes it tick, why it is going to be a good hold for the long term, and how she can always keep it secured. This is a jet age and you will be surprised at how these kids can quickly catch up. At least in such case, a paper wallet will even do, but at least make sure she understands how well she will need to keep it secured.