Short answer:
As early as possible and NO it is not dangerous for kids to start strength&conditioning in a young age.
Long answer:
A kid's peak of improvement for motor skills is between 10-13 years (Baur, et al., 1994), that means it makes sense to already work these skills in this period and even to start learning them earlier (7-9 years) (Weineck, 2000, p. 371).
The general science is in consensus, that strength & conditioning with kids, does not have a negative impact on their health and/or growth.
Often the injury risk is even lower, than in "normal sports activities" (Menzi et al.; 2007).
BUT it is important to not just adapt an adult program to kids. The most important factor is the movement quality, not volume nor intensity. The goal is not to stimulate hypertrophy or maximal strength but to work on a neurological and coordinative level to improve strength and basic movement skills
Following the LTAD (Long-term athlete development) (ENEPS, et al. 2020. p.31-35) the (age 6-9) kids learn the "FUNdamentals", basic motor skills in a playful way.
In the "Learn to practice" age (8-12) the kids learn basic movement patterns like (upper body push&pull, lower hip hinge&knee hinge, core stability). The movement quality is key factor, in order to be able to build on that foundation in their following career.
During puberty "Train to train" phase, the athletes continue to perfect the previous gained skills (maybe have to relearn some movements, due to growth peaks, which throws off their coordination)
Here is also the start of slowly implementing more resistance into the training.
Methods like maximal strength are only implemented after the age of 15 (depending on the biological age) (Zawieja & Oltmanns, 2016) which also starts the important phase of the athletes: "Train to win" phase.
Conclusion:
For an optimal development, it is essential to start early, using THE RIGHT TOOLS, with strength & conditioning programs WITH NO DANGER to the young athletes.
Sources:
Baur, J., Bös, K. & Singer,R. (1994). Motorische Entwicklung. Ein Handbuch. Schorndorf: Hofmann.
ENEPS, COSL, LIHPS, & Sportlycée (2020). LTAD-Lëtzebuerg lieft Sport. Rahmenkonzept. p. 31-35)
Menzi, Christoph & Zahner, Lukas. (2007). Krafttraining im Kindes-und Jugendalter. Zeitschrift für "Sportmedizin und Sporttraumatoligie", 55(2), 38-44.
Weineck (200, 2003 et al.). Optimales Training. jetzt: Balingen: Spitta
Zawieja, M., & Oltmanns, K. (2016b, March 9). Kinder lernen Krafttraining. Philippka.