I'm a Registered Dental Hygienist so I can let you know a little about what can happen :). Most people are right that they will probably be sedated. Mostly this is because kids are generally nervous and might not sit still. If the cavities are deep enough, like in the nerve then they will most definately need to be sedated so the dentist can work properly. It's true that in children though that if the cavity is small enough the filling can be done without anesthetic as the drill does not go near the nerve..but that would be a call that you would have to make depending on how easy going your 4-year old is.
Contrary to what was said below, cavities come from frequency of sugar consumption, not from minerals not being allowed back into the tooth. Minerals not being allowed back into the tooth certainly won't help the situation, but it's not the cause. When sugar is consumed frequently throughout the day, whether it's through food like doughnuts, Little Debbie snacks, candy, etc. or with juice or milk...what happens is that the mouth constantly is in a acidic pH state and that is where the cavity bacteria thrive, they eat the sugars and their byproduct is the acid that breaks down teeth resulting in cavities.
However, a tiny cavity can remineralize so that there is no more cavity mainly thorough special toothpaste you get at the dentist...I'm sure there are other ways too for those that are more into the homeopathic remedies. However, if the cavity is deep, and especially if it has gotten to the nerve, it will not remineralize and will continue to get worse until it's taken care of.
Again, depending on how deep the cavity is and which teeth are affected sometimes it is ok to let the baby teeth fall out naturally. If he/she is getting ready to loose the tooth and it's a small cavity, then it's ok to let it stay as the child is not in pain and the cavity won't affect the permanent tooth. however if the decay is deep and the child won't lose the tooth for a few more years the tooth needs to be filled to 1. Make it so the child won't be in pain, and 2. Make it so the decay does not affect the permanent tooth coming in under the affected baby tooth.
So you could use the tooth soap and it may help with remineralization, but if you or your children have a high sugar frequency they will still get cavities, probably even more so because there is no fluoride in the tooth soap.
Some of it also depends on when they were in utero...some Moms like to drink bottled water which is great, but bottled water generally does not have fluoride in there, so as baby's first teeth are developing in utero, they are not incorporating fluoride into their baby teeth and become more susceptable to cavities. Couple that with high sugar frequency and you have a recipie for multiple cavities.
I hope this info helped, and wherever you go the dental hygienist and/or dentist should be able to provide nutritional counseling so this can be prevented in the future. If you wish to not use fluoride there are ways to prevent cavities without it's use...mainly controlling how often their oral cavity is in an acidic environment by limiting sweets/juices. HTH!