This is such a tough subject sometimes, there are so many philosophies and opinions out there - just who do you listen to?
Do we follow the attachment parent theory? Where we never leave babies to cry and we have them with us all the time, or do we teach them independence at a young age - leaving them with other people and leaving them to cry it out?
Personally I think it's all about balance and learning what works for your family and your child. Let me share about my eldest child - she will be 8 next month, she cried for the first year of her life (Oh I wish I had discovered baby wearing and slings then) and she has always been clingy, she wouldn't even sleep at my Mum and Dad's (who live only 5 doors away).
I had lots of comments like if I sent her to school she'd get confidence because she'd be used to being without me etc and people in general seemed to think that by not sending her to school I was doing her an injustice and she would never gain any self confidence - well I just generally kept my mouth shut and continued to do what I thought was best, which was not leaving her in places she didn't want to be (unless there was a good reason) and allowing her to feel secure with us, knowing that we weren't going to make her stay somewhere she didn't want to.
In the last few months her confidence has just blossomed - she has been wanting to stay at Nanny and Grandads overnight (which we have allowed) and she even sang a solo at a christmas carol concert (she would never even stand at the front with other children to sing) - I was so proud and have the moment videoed on my phone. She also spent the entire day Sunday with another christian family (with two older girls) and had the time of her life from what I have heard - and she wants to enter a talent contest!!!!
Now how did this change occur? Was it because we forced her to go to school and spend half her day without Mummy, or made her stay somewhere she didn't want too in the name of teaching her independence? No, I truly believe it's because she's had the security she's needed, she knows we're there - I also think that if I had sent her to school she would have just done as I did and learnt to hide herself behind others and her confidence would never have had chance to develop (I never had much as a child) but because she's been home here with us, we've been able to see opportunities and encourage her slowly until she's gained the self confidence she needed!
All children are different - even in families and she is the one who's suffered with lack of confidence the most - her 3 year old sister is completely different and would go anywhere with anyone and sleep at any friends house lol (which brings a whole different set of challenges)
but I just wanted to encourage you if you do have a child lacking in self confidence, trust your instincts and know that you're not hindering your child by letting them stay with you and not forcing them out into the world on their own! The difference in my dd in just the last few months amazes me - she is certainly growing up and with it - gaining confidence which I thank God for daily!