When we have cruised with our son (13 months-36 months) we would often have him join us for the first part of dinner. We would have the server bring his kids meal while we had our first course - he would be with us from about 5:45-6:30. Then we would run him up to the nursery from 6:30-7:30 so we could enjoy the rest of our dinner in peace and he could play. This worked really well for us because a) he needed to eat dinner anyway and b) we didn't end up using all of our pre-booking nursery hours on dinner.
In addition to the time you have, we also usually blocked off an afternoon for our son in the nursery so we can have some time at the adult pool...but thats just us.
Also, we have left our son on the ship to do shore excursions a number of times (some
DCL sponsored and some on our own). We always let them know where we would be and gave them a way to contact us (cell phone with international calling available)-this isn't a requirement, its just something we did to make ourselves feel better. If you are off the ship the only reason they would try to access you is for a medical emergency, so I don't think you would need to have your wave phone on you for a DCL excursion on CC as a pp stated.