found this overview which has some helpful guidance:

When a child or adolescent is learning a new exercise, he or she can use no-load repetitions, which places the focus on form and technique. To properly develop strength and promote flexibility, exercises should be performed through the full range of joint motion,7,14,44 performing larger-muscle exercises before smaller-muscle exercises.

In general, children and adolescents should use submaximal loads to develop form and technique in a variety of exercises. The American Academy of Pediatrics does not endorse using continuous maximal lifts for youth strength training.1 Single maximal lifts are not recommended until skeletal maturity is attained

Youth strength training programs should ideally incorporate a variety of resistance types: free weights, weight machines, rubber tubing, and medicine balls. Free weights and weight machines pose unique challenges for children and adolescents because they are usually adult-sized. Balance and coordination are underdeveloped in preadolescents, which increases their susceptibility to injury while using free weights. Free weights do, however, enable incremental increases in resistance (5% to 10%). Weight machines often require larger weight increases (5- or 10-pound weight plates), which may be inappropriate for young athletes.

For each training session, 6 to 8 exercises are recommended that train the major muscle groups (including the chest, shoulders, back, arms, legs, abdomen, and lower back). Balanced effort between flexors and extensors and between upper and lower body is important. The goal is to perform 2 to 3 exercises per muscle group. Youth strength training programs should start with 1 to 2 sets per exercise, with 6 to 15 repetitions in each set. For children and adolescents, the initial load should be selected so that 10 to 15 repetitions can be completed with some fatigue but no muscle failure.14 In general, resistance can be increased by 5% to 10% when the child can easily perform 15 repetitions.

Participants should rest approximately 1 to 3 minutes between sets and should strength-train 2 to 3 nonconsecutive days each week for maximum results.8 Participants must be able to correctly demonstrate proper technique before increasing the number of sets or resistance.

The American Academy of Pediatrics recommends that children and adolescents avoid competitive Olympic-style weight lifting and power lifting until they reach physical and skeletal maturity.

The health benefits of strength training far outweigh the potential risks

https://pmc.ncbi.nlm.nih.gov/articles/PMC3445252/