Start by choosing something you FEEL good in. If you don't feel good in an outfit, you will be focused on things you don't like about your outfit during the session. 
 

For colours, I suggest neutral colours. Whites, creams, pale pinks or blues, deep navy or forest green. Steer clear of neons or graphic T's. That being said, these are your photos and your memories, so dress, YOU. 
Aim to compliment each others outfits, and not fully match. 

Try to limit patterns to one person/one item 
 

If you choose a dress I usually recommend midi to max length to avoid tugging your dress down during the session but if you're going for a smoke show look, go for the mini!
 

For little ones, in the warmer months remember bloomers/covering under dresses and over the underwear/diapers.