Have you seen this?

You won't find that on Duolingo at least, the only things you can turn off here is the talking and listening. If you sit down with them and do the typing for them, explaining the letters, corresponding sounds, and basic grammar, they'll probably pick up on both Swedish and writing pretty fast though. :)

Once they know a bit more Swedish you could look for some children's audio books in Swedish and let them listen to them.

Is there a Swedish speaking parent available? If so, this is the most obvious and effective way to teach your kids English.

If not, things get harder since you are going to have to give your kids reasons why you are teaching them a foreign language. Are you perhaps moving to Sweden soon?

If you have a mobile device, you might be able to find some cute vocabulary building games on the platform's store. I found a bunch of Swedish vocab games that were clearly geared at preschoolers on the iTunes store. They're mostly of the "tap the müs" variety, though I bet they might require some degree of reading.