D'Friesens have some of the best Low-German resources for children out there! I love their Sunday School materials, which I use on the daily, as well as their book series titled "Jieda Dach met Jesus". Each book is a collection of short stories where a Biblical lesson is learnt. The books are extremely affordable, especially if you order them directly from them.

They also have a very helpful tool on their website where you can find many different Low-German Songs simply by searching the name of the song in English or High-German.