Do not wait and try to make your own assumptions. If not sure, call anyway. Better to be safe than sorry. Next check their airway, breathing and circulation. If they spilled poisonous chemcials on their clothes, immediately remove all the clothes and wash the skin thoroughly, making sure before you do, there are no chemical burns.
Soap can burn the skin even worse.
You could do more harm than good. Do not give them any medicines to bring up the chemical. To do so can cause burns in their mouth and throat. If the poisoning happens at someone else's house, make sure someone didn't use a chemical bottle and replace the contents, making the label read the wrong ingredients. This can be very fatal if your child becomes sick and the hospital needs to treat him or her.