Paint the vanity a navy blue.

Find a stone yard and find a remnant for the counter.

Find an inexpensive sink. (Check Signature Hardware or the big box stores)

Paint the walls a soft white that works with navy.

Reuse the faucet.

Find inexpensive handles at one of the big box stores for the cabinet.

I would paint the walls a navy blue lol

New hardware on the vanity.

New organizers for the top of the vanity.

New mat for the floor.

Then take yourself out to dinner :-) and enjoy the break.

Inspiration:

Via Lido Soud - Lido Isle · More Info

Just paint. As cpartist said paint the walls soft white and the vanity navy blue. Done!

Navy vanity...what age are the kids? I just picked a generic picture. Since you have the hooks on the other wall, do you need the towel rod over the toilet? Thinking a larger picture looks nicer.

Ooooh, I love a navy vanity! Go with that, white walls, new sink and counter (stone yard remnant), new cabinet hardware, light, and faucet. If you get a remnant for the counter consider a single hole faucet or a widespread instead of the 4”. Also, paint your mirror frame navy to match the cabinet. Ben Moore’s Hale Navy is a beautiful true navy blue. Lighting, faucet, and hardware suggestions would depend on the style you want (traditional, modern, I like to mix them in a space 🥰).

Don't forget to change out the shower curtain. Depending upon the age of the kids, they might notice that and nothing else.

Yay!! Thanks for all the comments!! So helpful. What would plan B be if I DONT do navy? So pretty but not for me with three small kids (I can picture toothpaste dripping down)!

What color would you like to see? I photoshopped it, can easily change to any color at all. :)

My default is white but my design capability is zero :) What do you suggest?

Here is white....is the floor and existing countertop white or off -white? Although, if you only have a week, painting the walls and changing the shower curtain are the two easiest and quickest ways to make a difference rather than painting the vanity. Do the kids have any particular colors or things they like? Boats/Flowers/Firetrucks/Beach/Etc? Are they in the bright color stage still? We can look around for a curtain, then pull the wall color from it. I can easily mock it up.

3 little kids and $500. Paint the walls. Prettier shower curtain and towels to match.

Painting the vanity with a nice finish seems like a major chore to me. The walls would be a much easier project. I like Jean’s idea of large art over the toilet instead of a towel rod if one isn’t needed there. I think I would start with art and choose colors for walls, towels and mat from there.

I would do navy anyway with a high quality cabinet paint. They won't be little forever and toothpaste wipes off! Or skip the paint all together and do fun knobs--but white isn't doing much for me over the current wood. I'd go with new paint on the vanity and the walls and new art, shower curtain, towels, and rug. That will land you pretty close to your budget. If you have funds in the future, replace the vanity top too (if it's a standard size, you can get nice ones from a big box store). But that will blow your budget, especially if you need a plumber to connect the sink and if the mirror is integrated into the backsplash, so might be a project for another day.

Walls painted white and the mirror frame navy blue. Get a nice rug with white and navy for the floor.
If you have the time and energy, go to a stone place with your measurement of the countertop and a new sink and faucet. It will be really inexpensive and very childproof. Stone will last forever.

I use canvas art in bathroom - easy to wipe off. What are their interests? or a color? or vacation location?

Have they ever complained, or mentioned, anything they didn't like? If so, that one thing might be changed.

Ha! I think it looks fine for kids. Work on something for YOU while they're at camp. :)

Here’s a an idea for wall paint, art and shower curtain. Lots of colors to choose from in the shower curtain for a mat and towels.

Here’s a reversible bath mat that’s offered in many colors. Also on Amazon.