Hi Patty, the first thing I have to say is don't freak out!

It's only the beginning and it's better to see the mistakes now than later. One thing that helps me is to do a color study, just a small one. That way you can play with color and value without using lots of time and paint. You said you have a new canvas ordered. Why don't you divide this one up and use it for color studies? Maybe even do a black and white value study. I did one at a plein aire workshop last summer and it helped a lot when I got to the painting.
I agree with Marvin about keeping the values correct all through the painting, it can be difficult to lighten something later on. Having a black and white copy of both your source photo and your painting will help keep the values where they belong. I just convert both in paint shop and compare them side by side on the screen and mistakes pop right out.
Another thing I do is keep the paint smooth at the beginning, it's easier to fix drawing mistakes when you don't have to fight with ridges. I try to check my painting for ridges at the end of each painting session. If I miss some I scrape them off the next time I paint.
Doing a careful drawing helps too. And when I transfer I use acetate, and I save the acetate to keep track of the drawing; sometimes it gets away from me! So if I'm having problems I lay the acetate over the painting to check my drawing. I think you might find that helpful in keeping the reflections (and hip sizes, etc.) where they belong.
A T square would be handy to keep your horizon line straight.
The blue around the reflection...... I think it makes the water look deeper than it is, in the photo you can see the warm of the sand through the water. I would connect the reflections of the three kids on the left, they run together a little higher than you have it. I would squint to get the foam the right value. Compare your painting with the photo while squinting.
I hope this helps.