Congrats on your new little girl. I would have the boy fixed first, around 7-8 months, you will stop lots of bad behavior, like marking. Yorkies usually need some baby teeth pulled when they are fixed, but I don't know about Doxies. She should be fixed around 9 months, and she will probably need to have some baby teeth removed at the same time. There are many low cost spay and neuter clinics available, that you might like to check out.
.SPAY-USA. You can also do a google.