My wife and I just came back from a stay at the Ziva, although we walked to the Zilara to check it out. We used a DSU and were in a pyramid suite. Being off season and during any school breaks, there were not many kids. Apart from babies, we never saw more than 10 or so kids at the Ziva. They weren't bothersome at all.
The Ziva property was much nicer, larger resort, with access to two beaches - the public beach that the Zilara is on, and the more sheltered cove that a previous poster mentioned. The private one was nice as the waters were more calm, and there were a lot of fish swimming about as you wade around. The Zilara felt much more claustrophobic in comparison. However, while both had a range of activities, the ones at the Zilara seemed more geared toward adults (Vinyasa yoga vs. regular at the Ziva, poker game, etc.).
Overall, we were very happy with the Ziva and would probably pick it over the Zilara again if we were back in Cancun outside of school breaks.