Hi Khatty! Unfortunately, where you & I live in December, sending the kids outside to play is borderline child abuse! It is STINKING cold in December! (But not as bad as January & February)
Obviously, I don't have any experience with two, but I'm thinking that this will be your easiest year of celebrating! Your son will still have no real clue about what is going on, but next year and certainly the year after that, he will and one day double celebrations will be protested. I feel your pain...now already!
My suggestion would be to have a family & super close friends party at home (likely cheaper and fewer kids to tear the place up) for both your son and daughter. Then, have your daughter pick one, two or three kids (and their parent) to join you at a special kids centre or swimming pool (dry your hair before going out!) After the "event" go to their common area, open presents and give them each a little bag (your daughter can help decorate them) with treats for them to enjoy at home (or there) - lunch bags are fun for little ones. Or take them to (ughh) McDonalds for lunch.
OR...arrange for your husband and son to go out for the morning and have a tea party with your daughter and a few friends (and mommies). Cookies (made ahead of time), Tea Cups, tablecloths, pretty dresses, etc. Gift bags could include pretty smelling handlotion & a brush set? I'm sure you could find some nice looking stuff at the dollar store. Ask the moms ahead of time if you can paint the girls nails at the party. Unfortunately, this idea means cleaning the house before and after...and more leg work to pull together. But there is potential for it being a CALM day.

Just throwing things out here....no idea if they're realistic....