We just discovered these FREE piano Lessons for Kids which Ronan has worked through pretty quickly. It’s good beginner set of lessons with more to come. You pay for the paperwork downloads if you want them. For us it’s a nice “extra” to go alongside the Aventus lessons (which I am paying for).
Classics for Kids is an old favourite.
StringsLessons gives links to some good Youtube vid tuition for both ‘cello and violin.
Now I have to warn you that I don’t think online freebies or the ones you pay for will be enough. While there seems to be a way of learning just about anything from Youtube, there are some aspects of playing a musical instrument that I think will mean in the end you will need a live tutor, either via skype or in person. There are things like hand psotiion for piano and keyboard and how to balance a bow for strings that I think may need someone to show and correct personally.