I applaud you & your wife.
My kids ride down to roughly 32F (0 Celcius) on a back seat or TAB, down to probably 25F (-4 C) on a trailer with the rain cover on (significantly reduces wind chill factor). I use full face balaclava and ski goggles on kids and many layers of clothing (double gloves for example) if they are on the seat/TAB,
but we don't need for so much clothing inside the trailer - they are quite toasty inside there, but they can't really hear me and they grow bored.
Also one thing to look for is that cover becomes brittle when it's really cold.
I'm really impressed you can keep yours happy for 10 miles - my 2.5 y.o. daughter can make 4 miles, but during 6 miles she wants to jump out of the trailer, even when supplied with toys and company.