The train from Blankenberg to Bruges is a good, cheap, reliable option - we've done this twice before with young kids. There is quite a walk from Bruges station to the centre, but it's actually a really nice walk and easy to find your way to the main attractions.
I'd also consider taking a shuttle taxi directly from outside the port to Bruges - we were forced into this option on a Princess cruise (with rather expensive shuttles to Blankenberg!) that arrived very late into Zeebrugge - but we'd definitely do it again as it was much quicker and more convenient, straight to the centre of Bruges. We shared a shuttle taxi for 8, of course we had to agree on a return time with other passengers but it was no bother - I think the cost was only 60 EUR each way total, so our share for 4 of us was 60 EUR return + tip.