$100 is a good buy.
Headphones I would get but a coil cover I wouldn't. While the coil cover itself doesn't weight very much, the 'felt weight' at the end of the rod on the coil is quit noticeable. This can be especially fatiguing for younger people, an the search coil should be operated just slightly off the ground, by about an inch or so, so a coil cover serves very little purpose. If anything, not knowing the age and size of your grandson, you might want to consider a 'peewee' lower rod, if he's short.
I got three Coinmasters to my ex-wife for her three Grandsons for Christmas but need to round up two peewee rods for her as the two younger boys are just a bit shorter. I wish I would have found a clean, as-new Coinmaster for $100 as that would have saved some change compared with tree of them at my dealer cost. It's not a detector that I would want to have in my regular-use detector battery, but they do work and can find coins, as you noted.
One of the earlier version Coinmasters, in black or blue, would not have been a good buy for a hundred bucks, but the current issue Coinmaster was. Congrats! Now, don't forget to take your grandson out detecting as often as you can.
Monte