First, a piece of advice. Don't turn your bedtime narration into a job, or it will quickly become stressful for you and unenjoyable for your children. It should be completely okay (and I guess it is for your children) to read from a book, if you don't have an idea for a (new) story every night.
Now, to my answer.
Few professional writers have to come up with novel ideas every day (e.g. those that draw daily comic strips), and they usually spend the whole day on generating that idea or even work in teams (e.g. late night show writers). You, on the other hand, only have a minute or so to collect yourself while your kids lay waiting expectantly, so you have to make use of what's already on your mind.
There are some resources, such as fairy tales (that you can vary, expand, and adapt, as the storytellers of old have done) and the books the other answers have named, but I feel that if you begin to do research for your bedtime stories you are already on the way to loosing its fun.
So what I would do, and what I have done with my son a few times when he was young, is threefold:
Simply adapt anything that goes on in your mind to your children. There is no reason you cannot begin a story to your four year old with a dad leaving his home to go to his office. Or with a four year old being disappointed about not getting another ice cream (as your own child was that day). And then just let your imagination run. Let the dad's boss turn into a gnome and the dad have to catch him and return the office to order. Let the child learn that the ice cream was corrosive and has eaten a hole in the earth. Mix your day with the last book your children read or anything else, freely. I mean, Tolkien came up with the Hobbit because it was his job to study old tales, but yours isn't, so don't try to compete with him. If your job is selling cars, tell tales about selling cars. Make them fantastic by inserting elements from what you gleaned your kids watching on tv that afternoon while you cooked.
Make the narration interactive. Ask your children: What happens now? Did the dad catch the gnome-boss? Did the child fall in the hole? Then after their answer come up with what happens next. Children love to tell stories, they have ideas, so "make use" of them as co-narrators.
Read from a book, if you aren't in the mood to invent a story today, or even take a break and read for a few months, until the mood to tell your own story overcomes you again (if ever). Don't get into a competition with other parents about who is the best and most creative parent and whose kids have the most best fun life.
Any and all children's media that you already consume alongside your children anyway.
All the books, tv shows, radio plays, songs (!) – everything that you children read, listen to, and watch, and you are exposed to because you are in the same room with them, provide plenty of ideas for stories, characters, and topics that you can easily vary, mix, and combine with elements from your real life (even adult life, if you "downtune" them to the level of your children).