As Pennine says - unless they are keen to take the bottle (with either lamb or goatsmilk replacer, unless you have got the real thing) I would put them on calf pellets (you may be able to get some with high milk content, if not an 18% protein mix would be fine). But make sure they also eat good amounts of hay/branches/dry grass to ensure they don't bloat. If dried grass is available for horses (Readigrass or Justgrass in the UK) that would also be good for roughage/forage toogether with hay.
And start slowly and little with the concentrate...