DON'T DO THE MATCHY-MATCHY THING
You don't want to be all matchy-matchy (white shirts and jeans have had their time!) - but it does work if you pick three or four colors that you will all wear, and have different people wearing different amounts of it - for example, you could wear a coral dress with a blue necklace to bring in your little boy's blue shorts and a bit of coral on your baby girl's white outfit, etc etc. I'm more than happy to help you with this if you want some guidance - just send me what you're thinking and I'll give you tips/my thoughts.
No big logos or sports shirts - this is key - anything with writing or big pictures detracts from the subject (ie you guys!) and also dates quickly…
KEEP THE PATTERNS TO A MINIMUM
Try not to have too many contrasting patterns. One person wearing a pattern in the shoot is normally enough.
KNOW YOUR STYLE
Think about how you will all look together. If mom is wearing a maxi dress and sandals, it's not going to work if dad wears a suit and tie. Keep all of you in a similar style (and ideally make it casual) - the clothes that work the best are when they don't look forced, but just like natural choices for the activity that you're doing.
BLEND INTO THE SURROUNDS...OR DON'T!
Think about your surroundings - if it's an outdoor shoot remember your shoot will likely have a lot of green in. If it's at your house, bear in mind that most photos will be either in your master bedroom and/or your living area, so choose your clothes based on what will work in those rooms. If you have a bright pink bedroom, don't all go for muted orange tones because it just won't work!
That also applies regarding where you think you'll want to hang your pictures eventually. If you have a very muted colour scheme in your living room, it might be worth using naturals in your shoot so that the pictures, when printed big, work in your room.
Mom’s THE WORD
In my mind, the mom’s clothes are often the most important so it's a good idea to pick your clothes first and work out from that...you need to feel happy and comfortable in what you're wearing, so choose something that you feel confident in, both standing and sitting. It's my job to make you look beautiful, but it really helps if you're feeling confident and relaxed.
Lastly, and most importantly, don't overthink it. Wear what makes you all happy and comfortable and what feels right for your family. Show your family's personality and if that means crazy bright colors and patterns, then ignore what I've just said and do it. The most important thing is to try to visualize how you see the pictures turning out, and dress accordingly!