Nesting Blue Tits

Hi all

I am very fortunate to have nesting Blue Tits in my nesting box in the garden.  About a week ago, the babies hatched and mum and dad have been busily feeding the chicks relentlessly.

A few days ago, I noticed a Magpie hovering around and he then went to the nesting box and attempted to get in.  Sadly, he managed to get one of the baby chicks and peck it relentlessly - and then fly off with it.  I was horrified and desperately wanted to do something, but did not want to distrub the parents and babies.  I decided, as the nesting box was quite large and the entry hole was the complete width of the nesting box - to reduce the hole by attaching wtih strong tape (so as not to disturb parents and babies) some wood, enabling mum and dad to still come out, but preventing Mr Magpie to manage to get his head and part of his body in to 'kidnap' more chicks.  Mum and Dad are still happily feeding their babies, so the hole left is sufficient and the Magpie is losing interest now as he cannot get in their nest. 

How long is it before the babies will be flying the nest?  Also, once they do, should I remove the added wood to enable them more room to get out or leave it in situ?

Thank you for any help and advice.

Yvonne :o)