Hi Jenni, it would be good if you could try to build up your confidence, but I'm saying that as much for you.
Because in terms of your colleague.........well confidence isn't going to have anywhere near as big an impact on others as you're maybe thinking. She can be as confident as she feels like but there are a lot of other factors which are going to influence people just as much. And you clearly have plenty of those if as you've said now (and before!!) the children really like you and you've got a good relationship with the parents/caretakers. Of course it's probably going to be impossible for anyone to get on with every parent/caretaker but............
And regardless of confidence they like you/who you are e.g. you're caring, sensitive, listen to people, relate well to the children, want the best for the children when you're with them............right??!!
And you know what, plenty of people are going to like/prefer people a bit more "quietly spoken" if you are. Sometimes its easier to speak with people like that.
And as for your colleague's confidence, let it work for you. You clearly both have your own strengths, so maybe think about different things you'd like to do with/for the children and bring those strengths together. Ask her to be helping with some things, so she'll be bringing her confidence, you'll be bringing your own special qualities and experience (which are/is going to be so important). Maybe treat her more as a help in what you want to be doing as opposed to a threat. Afterall you have a lot more than confidence!!!
Although that's not to say I'm letting you off the hook in building some confidence
You do have a lot to be confident about.

Alison