We have a pram stoller with a boot and we will get one of those winter
sack things, but is that enough?

possibly not, the general advice is one more layer than you have on, which
in those kind of temperatures is going to be a lot of layers. It's also
helpful to protect them from the wind, so depending on the direction of the
wind, you might want to use a raincover even if it's dry.

I was thinking about waiting atleast 2 weeks. But If it isn't safe for

them to be out in the weather I will walk the dogs alone. Just
curious.

don't see any reason to wait even 2 weeks, in general babies are more at
risk from over heating than getting too cold, obviously you don't want them
to get hypothermia, but that is caused by prolonged exposure to cold, rather
than a hour outside, the baby will let you know if they don't like it!

Anne