I'm not the parent, but I'll reply too. I did SL 5x5 for a year, and then moved on to other strength programs. I'm young (20), so my results may not carry over to yours, but I found that SL was great for me. I definitely felt better after every workout, and in general felt a lot more energy. I felt sore after the first two workouts, but that dissipated very quickly. I've also fixed my posture (I've had TSA agents compliment my posture... not sure how I feel about that), feel much stronger day to day, and fixed some back pain.

As for tightness and injury... I think a crucial and under-stated facet of strength training is that when you are strength training, the pre-eminent concern is good form. Until you are competing and meets, there's no reason to focus strictly on lifting the highest possible weight. In fact, there is no time pressure, you have plenty of time to set up and rest after sets, no one is watching you, nothing to cause you to sacrifice your form. I just set up slowly, made sure I was confident for every lift, and tried as hard as I could to make each lift go through the full range of motion. It was a surprisingly slow-and-contemplative process (and workouts for me were about an hour each). As a result, I didn't find myself any tighter/less flexible, and I haven't gotten injured. I see a few friends get injured, but they also tend to be the friends who sacrifice form for weight-on-bar most often. There's no need to do that if you just check your ego at the door.

Point is, all sports can lead to injury. But strength training is naturally suited to making that easy to avoid.

I found that I burned out on P90X: the combination of a busy schedule already plus additional food prep time (I tried to follow the menu plan exactly) meant that I sacrificed sleep. And if you're doing intense workouts, sleep is very important. I could probably do it fine on a more relaxed schedule with an eat-healthy-but-don't-follow-the-meal-plan-exactly approach.

With Stronglifts, it's every other day for about 45 minutes including a bit of warm-up time. So far, this has proven to be a great pace for me as it allows me to still be quite busy with work, etc. while allowing sufficient time to recover in the off days.

Soreness is seriously not a problem. That's because you start with (the very humbling) empty bar, and add only 5lbs each workout. Eating is key-- the progression and ease of working out is noticeably easier when eating a lot of protein. But even though you're doing squats a lot, and exercises that hit your back, it actually feels good, like you're doing what your body was made for, rather than bad.

I used to be a competitive ski racer. p90x is basically the same workout we did for cross-training with more pull-ups and less weight training; it's a weird result. You get fitter but at least in my case you don't really feel awesome; you can just do things you couldn't before on the hill.

Weight training actually helps prevent injuries. If you don't have that as a part of your routine, you should add it.