A girl's flannel housecoat, a loose and light but warm item that buttons up the front, is very much a thing of the past. You might be able to find some at shops that specialize in old-fashioned items, although you will be best off looking for flannel bathrobes. If what you really want is a housecoat, your best bet will be to get a vintage pattern and sew one. Ideally, you will also sew matching pajamas and trim a pair of slippers, creating a very cute outfit for lazy weekend mornings and cozy evenings.
The Look of a Girl's Flannel Housecoat
Housecoats for children only began to be worn around the 1930s and went out of fashion as such in about the mid-1970s. A dressing gown or bathrobe, which tied in front, was more common for boys and often worn by girls, but girls were more likely to wear the coats. A girls' flannel housecoat was usually shapeless, long-sleeved and no shorter than mid-calf, though it often went to the ankle. It buttoned all the way from the neck to the hem and featured two pockets. It was a warm and comfortable item, but gradually deemed unnecessary as it became acceptable for a girl to be seen in her pajamas or nightgown. If a warm wrap was needed, the tie-front bathrobe - which could tie at either the waist or the neck - was found to be far more convenient. The housecoat has a retro look, but one rarely seen on children.
Shopping for Flannel Robes
If what you really want is a standard tie robe, you can find cute flannel robes for kids in any decent sleepwear shop. Sleepyheads has a large selection, including many that match some of the pajamas they offer. Rather than specifying flannel, however, they have cotton and cotton blends, as well as fleece and micro fiber.
You should also try the Bathrobe Shoppe, which has a good selection of simple robes in a variety of plain, mostly unisex, colors - with a few cute patterns as well.
For a classic flannel robe, try Grammie's Attic, which specializes in vintage reproductions. They currently offer a red Christmas robe with plaid piping. You should note that it is made of flame-retardant polyester and the chemicals used to create those fabrics have some attendant health concerns.
L.L. Bean offers a girl's fleece robe in several colors. It is child friendly with the belt stitched to the back so that it won't get lost. Customer reviews on the site rate it very highly.
Re-creating a Housecoat
If it's a classic vintage girl's flannel housecoat you're after, the best thing you can do is make it yourself. This can be ideal, as you can choose a better fabric than might be available on something ready made.
First, you'll need to find a vintage pattern. If you are an experienced seamstress, you'll know that you can get a larger pattern and re-size it, although this will be cumbersome. However, since a pattern this specific can be hard to come by, this may be your best option.
Start your search at Lanetz Living, which carries vintage patterns and frequently has something new to offer. You can also leave a request for something specific and they will contact you if it comes in.
Another great source is So Vintage Patterns. However, the prices can be quite high, which is great for a classic party dress but probably not what you had in mind for a housecoat.
If you can't find what you're looking for, post a query on sites like Thrifty Fun. Be as specific as you can - someone is bound to be able to assist you.