Sennosides (the active ingredient in Ex-Lax) is a stimulant laxative. With the amount eaten in a small dog there could be severe diarrhea accompanied by vomiting, nausea, cramping and abdominal pain.
If you don't have peroxide it is best to get her to an emergency clinic to induce vomiting.
But if you have peroxide go ahead and try to get her to vomit.
Once she finishes vomiting and empties her stomach to try and settle her stomach today you can give either:
1) Pepcid-ac (famotidine) at a dose of one half of a 10mg tablet per 10 to 20 pounds of body weight every 12 hours.
2) Prilosec (omeprazole) at a dose of one quarter of a 20mg tablet per 10 to 20 pounds of body weight every 24 hours.
These will reduce stomach acid and should help settle her stomach.
Then feed a homemade diet of 1/3 boiled, lean hamburger or boiled, white, skinless chicken, all fats and juices drained off the meat, and 2/3 boiled, plain, white rice.
If you don't have peroxide you can get her to an emergency clinic to induce vomiting.
If she doesn't vomit it back up or you choose not to take her to an emergency clinic still use the bland diet to try to settle her stomach and try to avoid vomiting and diarrhea.
If she is vomiting and has profuse diarrhea, then she should be seen on an emergency basis by her veterinarian.
Please let me know if you have any further questions.