Your save data is tied to your platform account; this includes the characters, your Collection, etc. You can delete the characters and create new ones, but your Collection will remain intact, even if you delete the characters. Quests that are only available once per account will also remain already done if they've already been done. Bonuses that are just granted once per account that have already been granted will also already have been granted. Deleteing characters isn't the same as deleting and remaking your account, so anything limited to once-per-account is still limited to once-per-account. Sadly, on PC that doesn't really offer license sharing, your only option if you wish to play completely and totally fresh from this point is to create a new Battle.net account and purchase the Complete Bundle. This will give you a fresh account, totally new. Right now, at this very moment, this is your only option if you want a completely fresh experience with nothing carried over from your nephew playing. With that fact, though, it should be noted that the Collections is for pulling items out of. So, if you simply don't go in and purchase items from your Collections and instead only play with things you, yourself, gain, then you'll effectively be playing a freshish game. - That being said, there's a major change in the works. Destiny 2, Curse of Osiris, and Warmind will be free later this year, meaning you can play the Y1 content without making a costing purchase. However, this doesn't include Forsaken or the Forsaken Annual Pass. This would let you make a new account entirely and not have to pay anything to play. However, to get Forsaken+Forsaken Annual Pass, you'll still need to buy the Complete Collection, which you can do right now and start fresh on a new account (as mentioned above).