Its a common occurrence for families to come with a suitcase of clothes to a shoot, unsure of the best things to wear. ;-) I know its hard co-ordinating, making sure it looks right... making sure everything flows well. I can sympathize and I'm here to help anytime. ;-)
In my studio I find lighter colors work well, with a mixture of textures and patterns. This look below works perfect for color and black/white.
Its best to not dress all the same. Every child is unique. ;-)