The younger children always have so much fun on the holidays. They usually have special events at school and church depending on which holiday it is. And as the kids get older, they like to do their own room decor.
If you start by picking up a couple items each year, it won't take you long to have all the seasons covered. Plus if you make your own it's even more fun, although not necessarily less expensive. Another good tip is to pick up things at the end of the season sales.
If you don't want to put holiday decor in their bedroom, the bathroom is an easy place to add seasonal items. A shower curtain, special toothbrush holder, a rug and maybe a wall decor piece. If you have room, something to put on the top of the toilet tank lid. If you don't have the money for that, just pick up some window clings, and add around the borders or corners of their bathroom mirror, or on the outside of the glass shower door if they have one.
Of course the shower curtain makes the most impact, and probably costs the most. But that is one item that is simple to make and you can usually pick up some inexpensive holiday fabric to make it from. If you're using a plastic shower curtain liner, just some cotton quilting fabric would work. Make the holidays a little more special for the little ones. They're pretty easy to please.