Traditionally our farm never tupped ewe lambs. Last year we tried a few, only 2 held and one lost 2 weeks from lambing, the other lambed herself and brought up a lovely lamb. This year we are trying a few again. Only texel X mules and only to a charmoise tup for easy lambing, and only those well over 40kg. I'm not sure what breed yours are, but they need to be at least 60% of their adult weight, not have genetic lambing difficulties, and go to an easy lambing tup.
Breed and size are critical.