My baby birds are small. Can I clean their nest without mom rejecting?

It is normal for the adult birds to press their stool up against the sides of the nest as they rear young, thus making the nest a bit "dirty". I would leave the parents to care for the babies without disturbing them as long as they seem healthy, alert, and are growing. Once the babies start to leave the nest more on their own, you can then clean out the nest as needed, and should clean it between every clutch as well. If the nest is extremely dirty or becoming dangerous, you can attempt to move the babies carefully to a new one (and then place it in the same area), but I would not do this unless it is absolutely necessary.

Yes, you may move them without consequence. Gently scoop them up with a bit of nesting material and place them in the larger nest box. Your hen may complain, but she will settle down once she is back with her chicks.

You can gently pick up the eggs if you need to move them or check them for fertility (small bird eggs will start out clearish and turn more opaque as they mature if fertile), however if you are planning on raising babies it is best to leave the eggs alone so that their parents can sit on them and incubate them without extra stress or accidental breakage from handling.

It's possible that is the baby bird poop. Most often the parents will take care of cleaning and can do a sort of technique where they press the stool to the nest wall to keep it "cleaner". I wouldn't disturb the babies until they are actively coming out of the nest and exploring on their own, which you can then use to remove the nest and clean it safely without injury a baby.

Keep the nest box clean. You can gently scoop the chicks up in a paper towel, clean the box, add clean nesting material and return the chicks. Make sure Noshi is eating a healthy. Ideally, she should be eating a commercial pelleted diet such as Exact or Pretty Bird. You can supplement with a fortified seed mix and fresh fruits and vegetables. Monitor the chicks for full crops (which means they are being fed) and weight gain. If necessary, a commercial hand feeding formula can be used to supplement the chicks.

You can hand raise baby birds if needed. It’s best to do as much reading as you can about the specific type of bird and what they eat. The parents will usually come back unless they have been injured. If you do end up taking the baby, she will likely be dehydrated so prepare a lightly sugared water to slowly syringe feed her. You will also need to keep her warm. Use either a red heat lamp or hot water bottle. Make sure it does not get too hot as a hatchling cannot move away from it if she gets too hot. You will need to hand feed her insects. Pick small worms like meal worms until she is bigger. I hope this helps!

Momma bird may be a bit stressed out now that there are new babies to care for. Adding in extra food and water dishes so that each bird has one to eat at without fighting, and making sure both birds have their own perches to relax on can help reduce tension. As for the babies, momma bird should care for them herself, however having some commercial baby bird food (most pet stores will carry it) can be useful in case she can't care for them all on her own. As they get a bit older and start to explore, you can start to offer treats and pets to get them used to being handled.