I agree with the others.
Its a bit too late in the year for piggies, particularly such young ones, to have time to get used to being outside (If they have only been kept inside up until they came to you) so it would be better for them to be kept indoors from now until next spring. You can then move them out next spring time so they've got all summer to get used to being outdoors and can move into the summer house next autumn/winter.
If being inside your house isnt an option, then I would definitely move them into the summer house now.
I would also recommend a thermal hutch cover as well as the blankets etc and tie a thermometer to the hutch so you can monitor more closely.
Being in the summerhouse will make a difference to the temperature My boys live in my shed (with a hutch in there also) and its not getting any colder than 19/20 degrees first thing in the mornings even though outside is 11/12 overnight. My boys are still free roaming the shed overnight and will hopefully continue to be able to for a couple more weeks yet. I'll then start to add in my warming measures once the shed gets closer to 15 degrees overnight, adding more as it gets colder - locking them into the hutch at night again, adding in extra hay, snugglesafes (I have four heat pads between my two piggies) and adding covers. I use a thermal hutch over, two blankets, a normal hutch cover and then a duvet over all of it.