One of the easiest hikes you can access without entering the Red Rock loop. First Creek is right after Oak Creek and before Bonnie Springs to the right, it's easily accessible from the highway. The hike is mainly flat terrain through the desert until you get to the more dense areas of First Creek, but even the descent down into the small waterfall and pond isn't as strenuous. Kids can do this hike no problem, as well as dogs.
I would suggest going a little after the Springtime (March-May) and even after monsoon season in Las Vegas (around July-ish) in the summer to see the real effect of the waterfall. The pool gets to a sustainable depth, and there's even fish and crawdads that are in the pool as well. If the trail is too bland, you could also jump off the trail and head into the more rocky, bushy area of it without a problem (you'll run into multiple streams and pools here). Occasionally there are burros to see passing through the area.
More than a beginner's hike (it really is a very easy hike), this place is beautiful during sunrise and sunset. One of my favorites to go to when I don't want to go too far away from town or I don't want to get too strenuous. This place can get busy on the weekends, so it's suggested to go on the weekdays in the mornings if you want to avoid the crowds!