In this family there is a lot of sharing between the kids. A tee bought for the little boy would normally fit well on the big sister too. Sounds a bit odd, but M is quite petite on the torso and D is quite a sturdy lad with a torso that could match his older sister so there of swapping tees is easy!
Here is a unisex outfit that I think would look great on both boys and girls (Native shoes comes in heaps of different colours too). The denims from Mini Nümph is originally for girls but there are no specific features that makes a boy not being able to wear them.