You are doing the right thing with feeding hay.
Do not be in a hurry to give them concentrates as their rumens will not be working efficiently if they have been starved. If you can get feed blocks, like Rumenco, high energy ones will help to give them a boost and encourage rumen activity, as well as supplying vitamins and minerals. If you can't get feed blocks, then at least get them a mineralised salt lick.
I would wait at least a week with feeding hay before offering them anything else. Then, if their droppings are normal and they are not scouring, give them some sugar beet pulp, which is high in fibre but will give them some additional energy. If any are scouring then keep them on hay until they have dried up.