Joining the Markham Museum with a family pass (2 adults, 4 children) will put you back $79CDN plus then, but then you can get free access to over 400 museums and science centres around the world through its affiliation in the ASTC Passport Program! To the other places around the world, each has its own rules--many will allow 3 or 4 children for free with 2 adults, so the rules are different for each place, but expect to save a LOT of money.
One warning--you CANNOT use it within 90 miles of either your home or the museum that you have the membership at, so no, you cannot use it to get into the Ontario Science Centre. ;)
For example, my wife went to the Corning Museum of Glass in Corning, NY--apparently, an amazing museum. Admission would normally be $19.50USD per adult and free for kids 17 and under. By having a membership at the Markham Museum, you just saved over $50CDN. Going to Buffalo for a day or two? Get in free to the Buffalo Museum of Science--adults are otherwise $11 and all aged children are $9--a family of 6 would save $58USD. Whether you're vacationing in NYC, Florida, Jerusalem, etc., you'll be thankful you invested in the Markham Museum!
Go to the Markham Museum website HERE. My understanding is it is sort of like Black Creek Pioneer Village.
Click HERE for a full list of locations around the world that you can get into through the ASTC Passport Program from May to October 2018.
Click here for the the ASTC website.