There is no rule or standard in this arena and is often debated. If you are going to charge for children, make sure it is in your house rules so when parents think they can show up with children in tow for free, they are aware that you charge (and then charge!).
Personally, I think having babies and children in a home are MORE work and have the potential to cause more damage, use more resources, and be less likely to treat my home with respect so I would be likely to charge extra for these tiny guests (if we allowed children at all). Other hosts are fine with a gaggle of kids running about and charge nothing more.
The choice is yours.