There isn't a one-size-fits-all answer.
They need protein to make milk, and there isn't much sustenance in the grass yet, and this year there probably isn't a great deal in last year's hay / haylage, either
On good grazing - once the spring grass really kicks in - fit ewes with a single lamb should be fine without cake. Some years, fit twin mums will be too, depending on breed of mother and lambs. Other years - and this is probably one of them - most will do better with a little cake.
On the hay front, generally sheep prefer good grass to hay, so should stop taking the hay when the grass is good enough. If they're queueing up, shouting and climbing over each other to get the hay when you bring it, they still need it.
If you set up a creep area for the lambs, you can probably withdraw cake from the mothers as the lambs start to eat significant quantities of creep feed - provided the grass is there for them.