Well, I think you need this rug I spotted on Amazon to go by the tub... Something like this (or paint them yourself) to go on the wall OR THIS When you look at those you will see links to beach signs, flip flop light string, outlet and switch plate covers, vinyl surfboard wall art and more. I'd pick a few of those, get bright beach color towels and accessories. Bathrooms are fun because it doesn't take too much to create a theme, but you can also go all out. I also like the idea of using a matching blue paint around the vanity area, keeping the tub area white ad accessorizing with the blue and then using blue and white towels or other brights. I like crisp blue and white, but not everyone does, and with a boy and a girl, I think they'd most likely like a lot of colors to make it fun and not too feminine or masculine.

My opinion (without seeing pictures of course lol) would be to use the blonde color. It hearing you describe the look and colors I think that may be your best. But if you have a chance to snap a shot of the hallway and your kids' bathroom, I think that would help greatly in picking the perfect color.

Will the color be used in the master bedroom also? Generally I think the rooms should be the same or close to each other.....but I am not a pro. Either a lighter version of the vanity color, or the lightest cream color in the tile might be a possibility.....

I'm sure both concepts could look good. I would go with a blue-green, but this is just my personal preference. I don't think there's a right or wrong answer here. However, I don't know if Niebla Azul is going to be the right blue. Since you are going to have beige elements in the room I would consider leaning more green. NA looks like it would be too cool. You need to sample in your light, against the finishes you've chosen in order to know which is the right color.