If you're tupping this year's lambs they certainly should be getting some hard feed. 1/2lb per head per day for a commercial-type. What breed are they?
Your multi-stock feed should be fine for the ewe lambs but you need to check if it's safe for male sheep.
Unless there's only puddled mud for them to lie on and nibble at, I'd say they'll be better off at grass, even if it's marshy, than in a yard. All our ground is what I suspect you would call marshy and all our sheep live out year round. Housing sheep brings its own problems, which you would be best to avoid if possible.
If you do bring them in, then other than the hard feed, they'll need clean straw for bedding and some hay if you are taking them off the pasture. Expect to feed four a small bale of hay over 7-10 days, assuming a good sized (25kgish) bale. Give them what they'll eat and clean up twice a day, if you can, otherwise they will waste a lot.
If you do decide to keep them out, offer them hay - again only what they'll clean up each time - and you'll soon know if there's enough grass for them to eat or not. I'd still keep them out, feeding a bit of cake and hay, unless they are getting muddy bodies from having nowhere dry to lie.