My mom's dog (my god dog) has the same problem with her back legs. Surgery is inevitable and very expensive but as long as you can keep the pain under control and as long as her little knee caps go back into place, you can put it off for a little while. Ask your vet about a payment plan. Sometimes they're willing to work with you, especially if you've been a client for awhile.

Good luck to you and little Maddie. It sounds like she's got a great mommy to help her through this.