I've never dealt with fleas on my rabbits.
But you can dust them with Diatomaceous Earth (DE) and that should take care of the fleas pretty quick. Just apply it to their fur every 3 days for 3 weeks.
Just make sure it's food-grade and that you watch out for their eyes/noses and be careful to not let them inhale it as it can damage their lungs.
Absolutely do NOT use anything with Permethrin on rabbits!
Some people use dog/cat topical flea treatments like Advantage but I wouldn't and it's not safe for kits..only adults.
I would just stick with DE
It's a powder and you can take a small handful at a time and rub it into their fur until it's all over them.
Just be sure to get the arms, legs, feet, tails and ears...carefully do the face. Try not to let the dust get into their eyes or nose