This cute girls's dress-up Princess Sofia The First Nightdress is a bargain price at Argos. It's been reduced down from £9.99 to just £4, and that includes a matching crown as well!
It's a lilac nightie with bows and frill detailing, and it comes in sizes from 18 months up to 5 years, with all sizes at this reduced price. The above link will take you to the smallest size, and you can use the drop-down size box to choose the one you want.
Stock seems very widely available in all sizes both for the free reserve and collect service and for home delivery, which will add a further £3.95 to your order.