My boys met children on plenty of occasions when on walks when they were around the age of 6 months, even some came up to pet them. They were relatively fine with it.
They don't get the same attention any more and now they aren't really fond of children, particularly their loudness/quickness is what gets to them. Socialisation is a big deal when it comes to shelties, it's something that needs to be practically constant. Mine went through a period of not being around children/dogs enough and I feel like they've regressed a lot compared to a few months ago. That's just my experience though.
They don't get the same attention any more and now they aren't really fond of children, particularly their loudness/quickness is what gets to them. Socialisation is a big deal when it comes to shelties, it's something that needs to be practically constant. Mine went through a period of not being around children/dogs enough and I feel like they've regressed a lot compared to a few months ago. That's just my experience though.