There are actually a few different tartans being worn in that painting, if you look closely.

The boy on the left is wearing the tartan that today we call MacDonald of the Isles Red (same as MacDonald of Sleat but with an added black line).

The boy on the right is wearing the tartan called MacQuarrie today in his coat. His kilt is in a different tartan, and I haven't looked closely enough at it to ID it.

Of course their hose are in the "MacMedic" tartan. :-)

Aye,
Matt