Tkachik entered a plea agreement and testified against his ex-girlfriend Engelica Castillo.
A jury convicted Castillo, Jada's cousin, and sentenced her to life in prison last year.
Tkachik and Castillo were supposed to babysit the toddler. Instead, the two were high on heroin. They beat Jada to death after becoming angry with her and then dumped her body in a swamp.