Museumkaart

Where are you seeing info about the Museumkart? Is it in a book or online?

1. As Jazz&Travels I don't understand the Museumkaart pass for the kids.
2. Just go through the booking process on the websites of both the Anne Frank Huis and the Van Gogh Museum and you'll see it is perfectly clear for users of a Museumkaart (EUR 0,50 per person booking fee for the Anne Frank Huis and free booking for the Van Gogh Museum). Don't forget your children when making a reservation, even when they have free entry!
3. The easiest way would be, I think, to buy the cards when you've arrived. The museum where you buy your Museumkaart counts as the first visit. So, do it at one of the museums on your list where you'll like to start your visit (except the Royal Palace, which does accept the card, but doesn't sell it).

Additional question. Since the card now has a 5 museum limit, I'd like to cherry pick and only use it at the expensive ones. I would like to pick it up at the New Church on Dam Square, to avoid crowds at busier museums. Is it possible to buy it there but not then use it to visit the church? Based on RS guidebook, I'm fine with the free gift shop view. Has anyone bought the pass (since the 5 site limit) at a museum but then not entered that site?

As I plan my April trip I'm finding the 5 museum limit, really limiting,

I think if you use it at the five most expensive museums you save 10 euros.

All the time I spent doing research makes it a not such a deal.

When you buy the card at a museum, it automatically counts as your first visit--you can't avoid that rule, so buy it at the first place you want to see. You can't buy them in advance--they must be bought in the Netherlands. I would not buy them for your children.