If it has to be one those, I’d say the dark gray. The light gray is just gonna blend in and look so drab.

Would you consider a white vanity with a gray and white counter? Use a subtle quartz. I would match the wall paint to the tiles.

My Florida bathroom

I agree with the above that white would look good. Or wood. Or a color since it's a kids' bath. I'd stay away from the grays; they will make your brand new beautiful bathroom look dated. The rest of your room is very neutral so the vanity could be almost any color.

How about blue?

• Avanity BRISTOL-V48-NB Bristol 48" Freestanding Single Bathroom Vanity Base in Navy Blue
• Avanity MODERO-V48-NB Modero 48" Freestanding Single Bathroom Vanity Base in Navy Blue

You could switch the hardware for black if needed.

thank you