As a parent I know this is always a stress point but it does not need to be {really}. And remember that the clothing will not be what makes your picture {its the people in the picture}. Here are a few items to think about to ease the process.
Determine where you will use pictures and the feel you would like (i.e. casual or formal).
Determine colors that make you feel & look good.
For group pictures make sure all are dressed in same tone - does not have to be same color - for example - pastels, neutrals, jewel
For senior pictures make sure it is comfortable to get a natural smile. Many opt for a few wardrobe options some examples also include sports uniform, bring instruments they play, college of choice shirt if taking after decision made, and some choose a casual and then more dressy option. Senior sessions are created for what you are looking for at Christina Bailitz Photography.
Avoid logos
Feel free to add patterns but if they are on multiple people make sure they are the same otherwise it will add too many dynamics to the photo.
Textures add depth & contrast to pictures
Don't be afraid to add an accent color on some of the family. We can always take some with an accent color item and some without.
And remember all your pictures will come in black & white too which will always look classic.