What would happen if you eat feces?

This would depend on the condition of what you ate but as a whole, a very bad idea.

There are many bacterial and viral diseases which you might contract, depending upon whose feces you are eating. Not to mention parasites. Parasitic worms are very common.
Human Feces is a "pure" waste product. Hardly any more nutrition left to utilize.
If you started on a diet of pure human feces, even if you managed to avoid diseases and parasites, you would in the end die due to lack of nutrition.
I also believe that your stomach would not be overwhelmed with your choice of food.

Babies and toddlers:
They crawl around and often put things in their mouth that they should not. This is mainly done because they use their taste buds in order to find out about things. Young children actually putting feces in their mouth is very common and normal.
This behaviour is not because they are hungry and actually want to eat, it is simply a part of their way of exploring the world.
It does put the children at risk of parasites and many children every year get treated for this.

Most children that have the ability to crawl around and taste various samples of .... whatever they find.... do develop better immune-system than children living in a "sterile" environment. This help them later on in life fighting other diseases.

Although we should not encourage children to taste feces, it will be difficult to 100% eliminate the possibility of this happening. It is nothing we should "punish" our children for. Just try to explain to them that some things are a "no no no".
To young children, it is often very effective to show them what they should not eat and make a face and a sound as in "this taste horrible". Children might not relate to the meaning of words but they do relate to facial expressions and sounds and in particular when one do this relating to taste.

Instead of scat-play try sticking to piss-play (urophagia) and maybe rimjobs as well as 'regular' sex.If u must be involved in scat do it externally only (coprophilia) and don't consume.Remember to clean up after!