Daughter was certified at 12, she's on her 2nd wetsuit (3mm) now at 13 (and 50+ dives later). Initially we thought equipment would be hard to find, but she's tall for her age, so almost all women's "small" gear fits her pretty well (except for fins, where she's already into size medium). She's using my older Atomic regulator setup, but we swapped out the mouthpieces (the adult sized ones made her gag). The one piece of equipment that ended up being the biggest adjustment actually wasn't anything size related - both my wife and I really dislike wearing our dive computers on our wrists (we both have them on our consoles), yet our daughter would spend all her safety stops holding and staring at her console when we set her up the same way - we moved her to a wrist mount computer and suddenly she was spending her safety stops looking around, enjoying the end of her dives.
Our son turns 11 in a couple of weeks - he's talking about wanting to get certified next year. He's smaller, so I'm assuming he'll need a kid's BC, but we'll do the same thing with the regulators (smaller mouth pieces, etc.). He already has fins/mask that he likes and is comfortable in... Given that kids grow so quickly, buying surfing wetsuits for them has worked well for us (they will outgrow them before the seams deteriorate). Much bigger selection of surf suits for kids... (assuming you're only diving warm water and get away with a 3mm suit).