This stuff is probably not familiar to you. It's only familiar to me because I live literally down the street from this hobby store which has all kinds of cool/random stuff. It's lichen moss, used in model train displays and you can buy some here online. This bag here cost me $9 from my local hobby store, which might be a better option if you have one in your town.
Anyway, open it up and break it into small pieces
Have your child apply glue to the cone in small spots
And then press the pieces of lichen to the glue to try and create an even layer around the cone
Do the same all the way around the cone, adding glue and then pressing on the lichen pieces until it's all covered
Now to decorate! Ooh this is the fun part! It was way harder than it should have been to try to art direct Henry's project. Have them add drops of glue and then stick the poms poms and sequins on the tree.
Cover the whole tree with decorations
Flip over a plastic plate and add the cotton balls in a circle, dropping a line of glue on top as shown
Place the tree on top of the cotton balls for a pretty snowy setting. Now you've got a lovely mini tree decoration for the table!