IDK, it just doesn't seem that 'off' to me. My first thought would be "get the police here" and I certainly would be running outside to look for my child.
Also you have to remember what a panic they must have been in after realizing Isabel was gone.
I have done that. My youngest daughter shoved a crayon piece very far up her nose past where I could reach it, and her hysterical crying was sucking it up further, I understand this was probably not life threatening, but I was a young mom alone with my girls, so I had my oldest call 911 and start talking while I tried to calm my little one down. I eventually took the phone and gave more information, long story short once she calmed down, I got it out with tweezers, called 911 back and told them we were fine..
Not nearly the same type of emergency, but still the same reaction of delegating 911 to someone else while you do other things (help the injured, search for the missing, etc.)
Sergio was just waking Isa up, we don't KNOW that he had his cell phone in his pocket, he could have been in PJ pants, he could have ran outside and told a son to call 911. I could understand him thinking "just GET THE POLICE HERE I will explain all the details when they arrive.." and not worrying that the person gives a perfect description or timeline, just getting LE out to the house to deal with the situation...JMO