I'm going to assume that while the children are undergoing accelerated growth, they're somehow being fed 18 years worth of thoughts, experience, language, and general knowledge. Because presumably raising an immature (mentally/emotionally/educationally/developmentally speaking) 18 year old is also unproductive for your time and cloning dollar?
In that scenario, I'd imagine Santa would exist as part of the simulated program that's fed into each clone to ensure that the clones emerge as self-sufficient 18 year olds ready to contribute to society. Either deliberately or through infiltration of the standard government training program (which may be only rudimentarily imaginative and therefore not include any official reference to Santa). Kind of like how Jane or the Hive Queen manage to infiltrate the Battle School training simulation in the Ender's Game universe, warping the standard content to serve a desired purpose.
Or maybe the cloning program does in fact produce fully-grown, 18 year old babies, who then need several years of care and attention from their biological parents anyways. In which case perhaps Santa delivers gifts for them until their biological age is 36 (at which point their chronological age, dated from 'birth' at the cloning facility, would be approximately 18).
Also, "In a world where raising immature [...] children is unproductive for your time" - You're talking about the real world, right?