1. Wear what you are comfortable in. Yes, there's a reason this is rule #1... if you feel comfortable, you'll look comfortable. If a t-shirt and jeans are what feels good to you, wear it.
2. Solid colors are great! This isn't a must... sometimes subtle patterns can look good, but solid colors are a sure bet.
3. If you have more than two people, make sure one person doesn't stand out too much... don't put everyone in dark colors and one person in light, or everyone in solids and one in patterns. Believe me, you'll notice later!
4. I love some of dresses the girls wear... especially with bare feet. It's always a fun summer look.
5. Simple accessories can make a photo fabulous! Try a cute hat, or a scarf, or fun tights, or funky shoes. Just don't overdo it with too many accessories, or accessories that overwhelm the face.
6. Don't stress about it! The kids might not stay looking perfect through the whole shoot anyway... and it might be those messy last images that end up being your very favorites! Let your personality shine through and have fun!