Honestly, I'm still looking at that. The jury is still out heh.
I trained in one art at a time, for the last 23 years, I'm not saying it's the right way, but it's the way I've done it so it's the one I know best. Ok, so I do blend them after I've learned them... And I've been doing MMA for the last 6 months, but initially, single arts.
I think there's something to be said for doing a single art, learning it well, then another art, then bringing them together. So that the person can concentrate more on the individual aspects before trying to combine them?
Again, I'm not sure this is 'the' way, just what seems to make sense to me. In my MMA for example, I don't have to even think about kicks as I've kicked for over 20 years, but I do have to think about staying behind my punches and boxing form as that's newer.
Luckily, my kids are young and have plenty of time for us to figure out which paths they'll take. My son likes the idea of MMA training, but at the same time I want to make sure that mentally he understands what he's doing and is careful with his tools etc.
Personally I like the idea of them doing BJJ simply from the standpoint that they can diffuse a situation without actually injuring an opponent. It's hard to do that with a round kick. (I know we also do joint locks, pressure points and many more, but not ALL the time). So I need to observe the kids BJJ programs and see, make sure that the safety nets are there, the understanding that if you PUSH this one, you can break and arm and that is NOT acceptable. I think wrestling coaches have known these things for ages and taught kids safely for aeons, I'm hoping the same goes for BJJ.