Hello Mirjam,
Having an artistic "muse" sounds to be a fascinating relationship...one that inspires an artist to do their best. I guess a sort of childhood "muse" might be a parent, relative or friend that encourages and supports creative aspirations.
My art professors always encouraged us students to keep our eyes and minds open to ideas from EVERYWHERE. Inspiration can be found wherever one looks and not just subjects to do with one's own field. An artist's mind never "rests"...we're always "on" absorbing sights and sounds around us.
Much inspiration comes from people watching, the decorations & clothing they wear, especially among teens and young adults....Japanese youth are well noted for their outragerous, colorful get-ups, so well known that trend watchers keep an eye on Japanese street wear and quite a few design books have been published recently about the clothing and lifestyle of these young people.
Interior design, high fashion, low fashion, fine art to outsider art, the artworks of the "untrained" or folk artists, picture books, magazines, etc...even words can be goldmines of inspiration, especially for me as interesting words, phrases will often conjure up all sorts of graphic images. Bottomline, everything and anything can inspire a new piece of art or design.
Many artists / designers also have large libraries full of books for research and inspiration.
Hallmark, the greeting card company is said to give their staff artists sabbaticals to visit galleries and other venues to keep their imagination working & inspired, learning new techniques and so on which in turn helps an artist to grow and keep their artwork from growing stale.
Designers and artists also look to their colleagues, admiring each other's works. For example, I might be inspired by another artist's cat designs.....thinking about cats might inspire ideas of creating my own cat figures either as natural cats or personified and dressed, flying cats, walking cats, riding cats...further thought might lead to cat associations such as cats & fish, cats & yarn, cat's & mice...these ideas can lead to oceans, mermaids, wooly sheep...in fact, I created an illustration for a canvas design titled Mrs Kitts Knits Mitts, an old fashioned dressed cat knitting mittens! The typical cat lapping a saucer of milk might bring out cows in a buttercup field or even a whole new idea of cows & their adventures!
Good ideas always inspire more of the same!