I have a lower ratio so I am not too picky about suits. Any full suit will do. Hats that fit over the ears (cannot tell you how many times parents bring stupid cutesy hats that sit on top of their heads
). Neckwarmers, mittens that are waterproof and have thumbs. If a child has a tendency to pull their mitts off I ask the parents to buy long elbow length mittens for them. Suits etc go back and forth but I have lots of spares if something is forgotten.
I am in Canada so we stay inside if it is colder than -17C for kids under 2. This year my kids are over 2 so I will likely go out to -21C no idea what that is in farhenheit.
For non walkers I usually sit them in the snow with shovels and pull them around on a sled and encourage them to crawl around. I do not take infants under a year.
I also bought a large shoe rack that I can place over my furnace vent to dry the clothes so we get out more than once.