Pinterest wants you to think that clothing is the most important part of portrait planning, but I disagree! The most important part is being relaxed and happy. Expressions and mood matter so much more than clothing.
In order to keep everyone comfortable and cheerful, choose clothing that is not itchy, uncomfortable, or out of character. This applies to all members of a family, but especially to children.
Make sure that the colors you wear look good with the colors everyone else wears. You could select a simple color palette and incorporate those colors throughout the group's clothing. A consistent color palette isn't essential, though, as long as everyone's colors work nicely together.
Consider the location of your photo session, and choose appropriate clothing. For example, if you will be walking on grass or rocky terrain, avoid high heels. If kids will be tempted by dirt, don't dress them in white clothing.
Items that you may want to avoid include clothes with writing on them, bold patterns, matching outfits, and anything that doesn't make you feel like yourself.