I'm so sorry - this is the problem with planning big parties for small children! You're really stuck financially and socially. If it rains, people just aren't going to come, frankly. 66 and breezy is okay - they can wear sweatshirts. But rainy? Three hours? 60 guests? Just what was your husband thinking? How much work was he planning to do to keep everyone occupied and entertained? Or is it all falling to you??
I'm not sure how you can keep twin toddlers happy for 3 hours with all that stimulation anyway! All I can think of is that you can spend a lot more money and go to a tent rental place to rent "sides" for the pavilion to break the wind and rain. You'll have to enlist the other adults in teams to handle set-up, clean-up (progressively - since the wind will blow things), figure out some sort of craft to do under the pavilion, etc. If there are tables, great. If they can be moved and you can do some sort of dancing or gross motor activity in the center to keep the kids occupied and moving against the cold, that might work. You said there is a big age range so I'm not sure what all you can do, but playing music like "YMCA" and having the kids act it out might cover different ages. You could do more old-fashioned things like the 'hokey-pokey' where everyone gets silly but all ages can do it and be laughing.
You need to enlist the help of your best friends/relatives, sit down today for a big discussion of how to make this work with your husband as an integral member and organizer, and get them to volunteer for different tasks. Many hands make light work - you absolutely cannot manage this alone, and you cannot assume that there will be any one activity that is appropriate for all these ages. Break it down into groups and hope for the best. Next year, scale back.