Leave the bunk bed wood finish as it is. Wood finish doesn't show dings or spots.

Pick new bedding for the kids. At these ages, you can give them a couple choices, no need to take them to Target or Walmart to stare glass-eyed at 50 kinds of kid stuff. A machine washable rug with nonskid pad would be fine. Declutter the chest of drawers when you get around to it.

I'm not sure about painting the bunk bed. It seems like a lot of work. But you certainly could. Here is an idea for you: A multi colored area rug, matching robin's egg blue quilts and happy fish sheet sets. You could select solid pillow shams - in different colors - for each of the beds, as well. The bean bag will take up serious real estate, but they will love it!

Here's a collage illustrating the idea. Hope it's helpful!

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What are their favourite colours? Interests? Do they share the dresser? Perhaps that would be easier to paint than the bed? You could paint their drawers their favourite colour or stencil their names on them. A rug would cozy up the space but I find rugs a pain to vacuum and prefer to minimize textiles in a bedroom. That's the only spot for the bed?

If you want to have a lot of fun with it, take the pink & blue colors in the rug that Manon showed and paint the girls portion of the bunk bed in the pink and the boys section in the blue. Leave the main body of the chest white and paint her drawers the same pink and his the same blue as the bed.

https://ruggable.com/collections/kids-rugs

These are washable rugs, ideal for kids. I've used one in our Kids' Kave at the vacation rental cabin I manage. Don't paint the beds. They'll stand up to a lot more action in the natural wood. I would shop second hand to try and find a dresser that matches them. Buy bedding and rug that work together. I think a valance will add color and softness to the window, yet be out of the way. Pick one in a color from the rug or bedding. Unless you decide to go for their favorite cartoon character bedding, leave the kids out of the selection until they are 10 or so.