I found a range of MLP clothing at Peacocks.  They come in sizes from 6 months to 4 years.

There's a pink 2 piece set, a hooded jacket and trousers in sweatshirt material, with a picture of Rainbow Dash on the top:

A long sleeved t-shirt with a pictire of Sunny Daze:

A denim skirt with a picture of Sparkleworks:

And a matching denim jacket with fleece sleeves and hood:

Judging by the tags, I think these are exclusive to Peacocks: