They might enjoy a bike ride over in the Waterland area just across the river from the central train station. There are a number of rental shops scattered around the city, but this place https://www.macbike.nl/ is located right at the train station itself, from where you can walk or ride the bikes down to the free ferry located about 100 yds away. From there you'll be on dedicated cycling paths (ie no vehicle traffic) for exploring the very pretty countryside ... a rural area with lots of little farms, cattle grazing in the fields, etc. One of the bike paths parallels the waterway (the Ijmeer I think) all the way up to the fishing village of Marken. There are a few little cafes along the way where you can fuel up with coffee, hot chocolate, etc. Would be a nice area for a picnic too - there are lots of scenic pulloffs available for that sort of thing, and I recall seeing children's playgrounds here and there that might be fun for the family.
As I recall the shops offer bikes with child seats at very affordable daily rates.
Renting a bike in the city is an option but would require you both to navigate thru city traffic to make your way to the ferry, and therefore might be a little nerve wracking with the kids in tow.