That's adorable BUT this is not safe. Rabbits carry a bacteria in their mouths that is deadly to tiels. And dogs can be unpredictable. My dog managed to get a parakeet in her mouth and gave it a heartattack. Please be careful when letting your pets be together.
It probably isn't good that my cockatiels always fly into his cage and forage in his litter tray then. I try and stop them but they're not the sharpest knives in the box at times. The rabbit usually free ranges but the cockatiels cage is on a stand so the rabbit can't get to it.