Ashtagon
Adventurer
It seems unlikely to me that you'd ever need to know a child NPC's exact STR, but you can work it out by considering what is average adult male STR, then -4 for every halving.
In traditional D&D, average adult male STR is 10. But in OGL Conan, AIR the mundane NPC stat blocks for 'Turanian soldier' and such tend to have STR 15. And Cimmerians get +2 STR. I'd suggest using either STR 13 or 15 for the baseline. Then if a child is 1/8 as strong that's -12. 1/6 would be -10.
You know, even with the soldiers having Strength 15, that doesn't mean the average adult, or even the average male adult, has Strength 15. Simply being a soldier implies a certain amount of bias selection and training after selection, which results in a higher than average Strength score.