Hi A.-
Decorating can be very exciting!! First, ask your daughter about her favorites and take her to the store with you to help choose bedding. Give her, say, 3 or 4 options. Next, take a pillow case or something with you to the store for paint. Find some colors that compliment (look good next to) the bedding colors. My one concern on this is that bedrooms need to be a "CALM" place. Soft, muted, greens, pinks, blues, yellows, oranges, etc. Bright and rich colors can make it hard for her to relax. If you decide to do pink and green, perhaps do a light sage green with a soft rose pink- or a soft baby pink. If you want to do pink and brown, which is also ULTRA CUTE, with polka dots, have her help you cut out circles of different sizes. YOu can either use a heavy paper and layer the colors, or you can take the sizes to a wood shop and have them cut some out for you. Dragonflies would be cute on those, too!
I am a fantasy and fairy tales/autumn colors kind of gal, so my first thought is a pale green room, fun bedding, and one of those nets that can hang from the ceiling and go around her bed. If she is a little too young for that, start small on something you can build on, where you only have to change little things, like the wall color, or what's hanging on it as time goes by. I love deep, warm, ambers, and muted greens with rustic decor. Dark, fuzzy comforter, deep, plush pillows...
For your son, since he is so young, stick to elemental stuff (trees, nature, or blocks, animals )so that it can be a learning place as well as a resting place. I saw in Babies R Us this bedding that was a soft creamy color with a light green and a milk chocolate brown. It had a fat little bird, a teddy bear, and a couple of other creatures on it. Very home made and very cute. Walls- pale green and blue. Again, it should be a calm place. If it looks like he is into more mechanical stuff, like trains and airplanes and autos, you can get some fun little prints to put on the walls, and maybe shelves where he can put his toys? Shelves can be re-used over and over again and changed a plethora of times as he gets older! You could even get a little bathmat/doormat in the shape of a plane or dinosaur... For the stars and universe stuff- get some of those glow in the dark stars and then paint or cut our planets and rings. Bedding can be black with stars, curtains can be sheer black or silver, or both! Or, go with a blue background/shades of blue and then make the planets and stars different colors.
My sister's little boy has rainforest stuff. Lots of frogs, birds, bugs, etc. And these can be toned up or down as he gets older and really starts showing his personality. The room is painted in soft blues and greens (2 walls of each) and the curtains are shades of blue. It looks really great and is really comforting.
As for doorways, if you ever have concerns about doors getting shut, etc., you can put sheer curtains up with a decorative rod...
All in all, I would stay as far away from wallpaper as possible. I love borders, but not for kids rooms. Too much work to get off unless you have the patience for a steamer and a scraper.
For paint, there is a new label out there that I saw at home depot, and maybe some of your local stores carry it. But is is an odor free, non toxic, indoor house paint that is supposed to be of the highest quality. The best part- no paint smell for days and days.
I encourage you to get dirty, get your kids involved, and just have fun. Regardless of whether or not it looks professional, what's important is that they are happy with it and that you all had fun doing this together. They need to feel that it represents them and they want to have a great time with you while doing it. So what if your planets are lopsided- name them! And if your daughter's room has an enchanted castle far far away in the background, get some fairy tale books and a fun nightstand to put them on for an evening story time. Creativity is in the desire of your heart, not in the talent of your hand, and I really think that you are going to do something fantastic with your kids!
Good luck, A.!
-E. M