You need to suggest a budget, which we will cheerfully ignore.
A lens that was frequently sold with the K-50 is the
SMC Pentax-DA 18-135mm F3.5-5.6 ED AL [IF] DC WR Reviews - DA Zoom Lenses - Pentax Lens Reviews & Lens Database. I thought this lens was a little expensive new, but it is often cheap used. I told myself I'd buy the first one I saw at $200 and the next day, I did. It's weather resistant, silent focus, and feels like a quality piece. The focal range is reasonable and really good for me. You can find lenses that go from 18 to 300mm and that sounds great, but they usually compromise somewhere. The 18-135 isn't perfect everywhere either, but it does "nice outdoor photos" for me.
I have a DA 18-55 WR also which is pretty good, and these are cheap. I still use it a lot because it's small and light. I don't need both - one reason to not get the 18-55 is that if you see yourself buying many lenses, the 18-55 starts gathering dust pretty quick.
Newborn shots suggest a prime lens, like the DA 35/f2.4 already suggested, or a DA 50/f1.8. The reason for those is to take shots without a flash or turning on every light in the house. Those lenses let in as much as three times the light as ordinary zoom lenses, and the baby will be in a fixed location for a while so you don't need zoom flexibility.
Maybe one more lens if you really have to, a version of the DA 55-300. That lets you zoom in to stuff, probably not quite as much as you want, but as much as you can afford for now.
It helps to stop after a few lenses because using them helps you figure out what you should concentrate on next.