The answer to your question depends on several things:
1) Does your family ride lots/all rides on the FP system? People can forget that not all rides at DLR are included with MP/FP, so if your family doesn't ride all the FP rides, then MP might not be a great fit for your group right now. Check the list of FP rides (look on the DLR website or on the app) to see if you'll be riding those rides.
2) Will you be using PP and getting ride photos? Not everyone cares about this, so if it isn't a big deal for you, then MP might not be such a value. If you do want PP, it can be worth purchasing MP for one person each day just for the photos.
3) Do you need to have MP every day of your trip? For a multiday trip, some families only buy MP for the busiest day/s, not all days. This can save some money. Plan to push hard on the day you have MP, then you can relax the next day.
Some guests try MP and can't do the parks without it. Others enjoy the parks just fine without it. We use it and really enjoy it. And we also have great park days where we don't use it at all. MP isn't magic in and of itself -- you'll still need good FP skills and organization to make the most of it. Read this thread to get a good understanding of MP (
Maxpass/Fastpass: an overview and some strategy), then decide whether it will be a good fit for your family and worth the money. Or would regular FP be just fine and the savings spent elsewhere be a better fit? There is no wrong answer -- only what is a better fit for your family right now.