A double back tuck timer is a standard drill on multiple progressive surfaces which begins a long time before the gymnast competes it. I've generally-to-always seen it introduced first on a bouncy surface as a timer for the bar dismount with the later intention to move it to fx.
I've noticed the earlier the skill is introduced, the worse its fundamentals can be long term. I'm not a coach, but young children may not fully understand how to set or keep their head in? Plus, there is little benefit to adding such a skill early on hard surface which means the sociological situation at play (ambitious coach, child, or parent or all three) can inhibit the development of long-term basics.
One of my children face planted a bunch at the US Challenge - Hopes in hard surface warmup shortly before her 10th birthday. The following year her double flip was better because she switched to pike and the heel drive helped her stay off her face, even though her set was still very poor (and was always poor).