If you do go to Skagway to ride the train, it is a cruise ship-oriented town, not an RV town. There are only 3 RV parks in that town and, unless it has changed dramatically, avoid Mountain View like the plague. Pullen Creek is nice, but right on the route from the cruise ship dock to downtown. That means that there is a constant stream of pedestrian traffic crossing the entrance drive into the park and walking 3 and 4 abreast in the street itself. Multiple trains also stop traffic on the only street that accesses the park. Only Garden City is, in my opinion, worth considering for a multi-night stay (which you may need if you are riding the train).
Two other camping alternatives exist. One is to stay at the Park Service's very isolated, dry campground at Dyea and commute 10 miles over a very winding road. However, it is the perfect place to stay if you and the kids want to experience what it was like to hike all or part of the infamous Gold Rush Chilkoot Trail. The other alternative is to stay in Carcross and commute on the highway over White Pass and go through customs in each direction. We chose a combination of a night in Carcross and a night at Garden City for the train ride and then the ferry to Haines. But it wasn't our first trip to Skagway, so we had played the tourist role previously.
I would highly recommend taking the ferry from Skagway to Haines and continuing your trip via the Haines Cut-off to Haines Junction. It is a very short ferry ride, will be something very different for the kids, and will give you a feel for what the Alaska Marine Highway System can offer on a future trip. And it's not like you will miss Whitehorse by doing that because you can't get back to the Lower 48 without going through Whitehorse unless you opt to take the ferry back home.
If you do go to Haines, as johnwalkerpa1 mentioned, don't miss Chilkoot Lake. Absolutely gorgeous scenery, great fishing, and wonderful wildlife viewing. In addition to the bears, the best bald eagle photos we got anywhere in Canada or Alaska were not at the Bald Eagle Refuge on the Chilkat River on the other side of Haines, they were at the outlet of Chilkoot Lake.
One night we decided to celebrate my wife's birthday at the Lighthouse seafood restaurant in Haines. The tables are close together and conversations got started between tables. One man told us about getting out to Chilkoot Lake at 6:00 AM to photograph a pair of bald eagles. He said that it was so productive that he was going back out the following morning. So we took his advice and left the Haines Hitch-up RV Park at 5:30 the next morning. And, sure enough, we got there a few minutes before he did. We took dozens of photos of two big eagles not more than 100 feet away, first in an evergreen tree, then down on a gravel bar in the river. And, if you wanted to dry camp right there instead of staying in an RV park in town, there is a beautiful campground right on the lake at Chilkoot Lake State Recreation Site.
Have fun with your trip planning! BTW, you do have the Alaska Highway bible (The Milepost), don't you?