Hi Dianne! It sounds like you're planning a wonderful family vacation. I can assure you that you will be able to meet the characters without spending a lot of money!
There are many places in each of the parks where you can meet and greet the characters for free. You will have to wait in line, but your children will each get a chance to get autographs and photos with a variety of characters. Some of my favorite meeting places are Camp Minnie-Mickey at the Animal Kingdom and Toontown Hall of Fame at the Magic Kingdom. Both of these locations offer opportunities to meet multiple characters.
Check a times guide when you visit each of the parks to see the locations for all of the meet and greets offered that day and the times you can enjoy them.
If you can fit it into your budget, I do recommend making reservations for at least one character meal during your visit. Breakfast is a good choice, as it is the most affordable. I would suggest
Chef Mickey's at the Contemporary resort. The breakfast buffet offers a wide variety of food and my family has always enjoyed the character interaction here. You can expect to meet Mickey and his friends, which makes this a good choice for both boys and girls. Your children will even have the chance to see the monorail glide by overhead!
I also love to get early breakfast reservation at
The Crystal Palace in the Magic Kingdom. This restaurant also has a great breakfast buffet that you can enjoy while meeting the Pooh characters. If you get reservations before the park opens, it's a great opportunity to get photos of your family on Main Street and in front of the castle without a lot of people in the background.
I hope I've helped! Have a wonderful family gathering at Disney World!