Please read the labels. The meds you are describing are the same medicine.
Robotussin and most store brands are Guaifenesin. Which is an expectorant.
It breaks up the goo and makes it easier to cough up. The med is made to cause them to cough to get the goo up. IF you want to stop a cough you need to give a cough suppressant.
Robotussin, Tussin, store brand name....all over the counter meds are almost all the same drug.
Here is a link to the information about it.
http://en.wikipedia.org/wiki/Guaifenesin
I suggest you read the labels to make sure then call a pharmacist. Walgreens is open 24 hours, there may be others too. A pharmacist can tell you if you are overdosing your child and they need to go to the ER.
Overdosing meds is the reason all the meds for children under the age of 6 were taken off the market. Children were dying. Check out the stories on the CNN website about over the counter meds killing children.
The doc can write a prescription for an actual cough suppressant that will be effective. Even the ones that do not contain Guaifenesin still do not have enough meds in them to do the job. There is very little scientific proof that any of the over the counter stuff actually works to stop coughing.
Tesslon Pearls work but are expensive. They relax the throat to where the cough can't happen. There are some meds that are narcotic in nature that will also numb the throat muscles and stop a cough. But be sure to give her something the next day to keep it loose so she can get the goo up and out. She will get better sooner if the goo is gone.