Can I use a 4x4 concrete post holder to install a gate?
No. Those holders give virtually no torsional support. Your gate will tear it right down. Plan on robust posts in a substantial concrete footing to carry that torque load. 

Either that and do what I did and run some snow fence on a pair of 1/4" vinyl-coated cable. The gate won't contain a toddler, and the snow fence wouldn't either, but it gives a visual barrier and slows them down. That's the main goal. The rest is handled by good parental discipline. 

You're going to find the toddler years whisking away in no time. Do you really want that big gate there anyway?