glad you have revived this question about Caylee's age, thanks for the thread.
KC had cared for Caylee without any harm ever coming to her until this point, through the infant years, the newly mobile years, the terrible twos, etc. A few friends and associates also cared for Caylee during these years, with no harm apparently coming to her by KC or any of these people. True, Caylee's speech was becoming more intelligible at this age, so she could pose a risk to anyone with a secret that she witnessed or was victim of. She could tell her grandparents or her mother about something that had happened to her or something she'd seen.
Of course if that is what led to her death, it would have had to be a secret bad enough to murder a child to conceal. In the case of KC, if she wanted to engage in any behavior she didn't want Caylee to tell the grandparents about (or if she wanted to be free for that matter), she need only let Caylee live with the grandparents and visit any time she wanted. Easy. And I do think she takes the easy route. Also, if the grandparents thought Caylee mentioned anything inappropriate like somebody hit me or somebody did whatever or Mom is doing whatever, I think KC would just say "she got it from TV, you're crazy", etc, etc. I think simple denial and evasion would be her style. I think KC might just leave Caylee with the grandparents at that point so there wouldn't be any more incidents to worry about.
Regarding killing Caylee to "be free", why now at almost 3 yrs old, why not before. Caylee was not far from being able to enter public pre-K now, etc. Casey could even have asked her mom to help get childcare until then.
Caylee's death could have occurred pursuant to a sexual assault. Older children might be controlled and used as an ongoing victim by telling them they will be killed or their family will be killed if they tell, but Caylee was probably too young to be threatened effectively in this manner, she might just be killed instead, or murdering the child might just be part of the perpetrator's m.o.
I am less inclined to think that KC or someone who had previously sat for Caylee murdered Caylee than another party simply because there was no history of harm by these people, then suddenly, when KC has new associates, new preoccupations, Caylee exposed to new crowds, Caylee gets harmed. That was the factor that changed. Caylee could have been murdered by anyone who was around, when her mother was busy with a boyfriend or friends, or on the computer or phone, or intoxicated, or left Caylee with someone, or whatever (by any associate of hers or by a stranger who saw an opportunity). Caylee's death could have been something other than a sexual assault, too, it could have been a retribution crime of some sort against her mother or her mother's associates. The tape on the mouth could have been a message to her mother or someone else.
Or her mouth could have been taped just during a rage attack by someone who wanted her quiet because they were sleeping or hungover or on drugs or whatever. A few of the possibilities anyway. I definitely agree that she could have been a bigger risk to someone of telling something at this age than before. I think if she had told anything on KC or was going to, KC's coping mechanism would have been denial and evasion, rather than murdering Caylee. Though I can't rule out that possibility either of course.
