In order to see how many children your pet will have, you have to do research. How exactly do I do this? xD It's been puzzling me.

You need to know the dominant muscle gene in order to see how many children your pet will be able to have. To research any gene, you go to your pet's page and then to the Genetics tab. There you have the option to research all genes (as long as you have enough silver and vermillion on hand) or a single gene. To find all your pet's genes at once you will need 2000 silver and 20 vermillion, 100 silver and 1 vermillion for each gene. If you don't have enough, you will research as many genes as you can randomly. Single gene research option will research a random gene from your pet (so its a 1 in 20 chance that the dominant muscle gene will be researched if your pet is totally unresearched), unless you have the Inn and a Geneticist staying there. He will give you the option to choose which gene exactly you want to know and then research that one for you.