If you already have the entire story plotted out and wrote based on that, trying to add in an additional 70,000 words is going to be near impossible, I'm sorry to say. It's going to sound forced and have a lot of filler.
A few things you COULD try, however:
-Think if there is any new twist/plot element you could add in to flesh out the story
-Take a look at the length of each of your chapters. Sometimes, just setting a chapter length goal will keep you writing until you have what you need, such as at least 10 pages per chapter, or 3-4K per chapter (with 25 planned chapters).
-Start from scratch, using the chapter length goal in mind. Scary, perhaps, but you may just find that your initial 7,000 words was a very in-depth outline!
How much you'll need to add also depends on the genre you are writing in. For middle grade, aim for 40-60K, for young adult aim for 70-80K, and for adult aim for 75-90K (very rough, generalized numbers, of course).
If you ARE writing in the children's genre, I can tell you that at 7K, as-is your novel could be a chapter book (but that would all depend on the subject).
Hope this helps! Good luck!