Actually, an origami car is quite easy but as you lack the practice, it may seem challenging at first, so don't lose hope. Here are a few links that I found to be extremely simple and easy to understand:-
https://www.youtube.com/watch?v=UsrrfUWqXto
https://www.youtube.com/watch?v=tSVl6G7u5aA
One of the links is a basic tutorial and the other one is a little more creative. You can try the second one once you get the hang of the first one.

Your kids are going to love origami designs and there so many simple structures that you can try. For example, dolls, birds, cars, etc. are extremely simple to make and later, when you do build a dollhouse, you can place the small origami items in the house. All you need is some practice and you will be building a dollhouse for the kids in no time.

I am an art & craft teacher at a local private school and for more than 4 years now I have been into origami. Today, origami has become an important part of my life with many decorative things in my house comprising of origami designs. I also have a 2 year old baby girl for whom I have made many origami designs such as dolls, cars, animals, etc. Basically, I have always had the urge to experiment with something new and that's when the idea of making a dollhouse came to mind.

Needless to say, I went all out for this one and got different materials and tools and explored different websites, tutorials, and books to get different ideas. I wanted it to be the perfect gift for my girl, so I slowly began to make the dollhouse room by room. From a small kitchen, dining room, bedroom, to the baby's playroom, it had everything that one would expect from a huge dollhouse. With different colors and textures, the entire thing appeared to be vibrant and when I showed it to my kid, she was all smiles. That little smile was worth all the efforts.