I would suggest that with the weather conditions you describe, plus ad-lib hay and a lick, any cereals you feed will just be to make the ewe lambs friendly. So no more than a very small handful/ a tablespoonful each per day. If snow falls or the ground is frozen hard then you could increase that very slightly.
When you say 'cereals' what do you actually mean? We feed Carr's Tup & Lamb, which is low protein but relatively easy to digest.
Once you are breeding them at 2 years you will need to feed them a little bit more in the 6 weeks to lambing, but they don't need anything like the quantities that large breeds do.
One thing which they would really benefit from is some branches to chew on - make sure they are from non-poisonous trees such as willow, ash, or orchard thinnings. They will get some essential minerals from the bark, and they love to browse as well as graze.