Not sure I would talk about mag declination at all to be honest. As the others have said, it's a bit complex for them and could just end up with them confused. Plus you have to remember, that in the UK the declination is only between 2.5 and 1.5 degrees (and getting smaller), it really isnt worth bothering with (here). The other issue is that map reading is really about relating the map to the land. If you can see landmarks in your environment and can relate them to the map, then you know where you are and dont need a compass - you only need a compass for orientating yourself and the map to the land - and you can do that just as well by looking for the mossy side of a tree, or looking at the way the sun is moving - and that is prolly tons more fun for cubs. Get em used to reading the land and relating what they see to the map, landmarks, contour lines, rivers, buildings etc. A good solid basis in that will underpin complex compass work if they later want to attempt things like navigating featureless landscapes in the dark.
Teaching them to navigate without a compass, is probably the best gift you could give them. If you really must do compass work, I'd stick to the Silva 123 method and teach em nowt else at all.
...and for your own reference, as said above, declination is "grid to mag add, mag to grid get rid" - for the northern hemisphere and the opposite for the southern hemisphere.