I’d imagine there were a mixture of ages working on digging the tunnels. Small kids were used for their agility at the farthest ends. But adults were probably not far behind in shoring up the work, lighting, ventilation, concrete and so on.
Based on that, I’d surmise the recent executions were of the adults; the ones more likely to provide intel to Israel, the ones more likely to have gotten a glimpse thru blindfolds going into the tunnels and able to make deductions of useful info of paperwork and logistics.
Besides, the kids will be needed later for suicide missions.
What the execution tells you is that there are more tunnels that still have not been destroyed.