I have three main pieces of advice for what to wear.
1. Your outfits need to be tonally similiar but it doesnt have to be the same colour. So all the same type of tone - dark, light, pastel, bright etc.
2. Choose a colour palette. In the example above its red, chocolate and grey. Try not to have more than three colours, but you can usually add black, white and denim and it still works.
3. Dont be too perfectly co-ordinated. And you really dont have to wear jeans and white t-shirts. If you wont go out for dinner in your outfits you have selected for your photo shoot then maybe you are just a bit too co-ordinated.
The example above would look great in the studio but would also look fabulous on location.
Extra tip : for small girls still wearing nappies, if they wear a skirt make sure to put some cute nappy covers or footless leggings on underneath so if they sit down you cant see the nappy in the photos.