let them eat as much timothy hay as they want. bunnies eat hay constantly. it helps keep their digestive system moving.
make sure they have plenty of clean, fresh water too.
if you want them to eat pellets, only buy high-quality timothy pellets and give them very small amounts. introduce fresh DARK greens in small amounts one at a time to see how they tolerate it, and go easy on the carrots as they have a lot of sugar, which isn't good for bunnies.