Is getting a semi large tattoo going down the side a bad idea meaning if you one day decide to have kids will it stretch and ruin your tattoo?

It all depends on where on the side the tattoo is. A tattoo on your belly will expand with the skin. What tattoo artists usually say is that you need to moisturize the skin a lot to keep it flexible, and you might require some touch up work after giving birth and when you're back in your normal body shape again.