Mary-Kate and Ashley Olsen are set to design a range of children's clothes.

The movie-star twins, who already run two successful fashion labels The Row and Elizabeth + James - have joined forces with US department store JC Penneys to create an affordable line of outfits for youngsters.

The range, to be called Olsenboye, is set to be released next spring and will be geared at an entirely different clientele to their two other ranges.

Earlier this month, it was revealed Mary-Kate and Ashley had designed a range of sunglasses, creating four vintage-inspired styles for their fans.

The designs - being launched in collaboration with top eyewear maker Linda Farrow - will be sold through their luxury label The Row.

The line - described as "sleek and sassy" - will hit the shelves in spring 2010 and will range from $325 to $390 each.

The collection will feature a classic aviator, 60s inspired round glasses, an oversized square frame and vintage inspired brow-line shades.

The 23-year-old twins, who have made oversized shades a signature part of their style, plan to see how the designs sell before expanding the range next year.