Typically, how many baby Blue Tits survive?

Hi,

We put up a bird box with a camera and much to our delight last week the Blue Tits that have made a nest in it hatched their eggs.

Of the eight eggs laid, seven hatched and the chicks have been tended to by both parents for the week and seemingly growing (with tiny discernible feathers coming through)

Sadly, overnight 4 passed away leaving only 3.
Is this normal or is the cold wet weather making things worse?

While I understand this is nature's way, I want to manage the expectations of our kids, so typically what is the survival rate of baby blue tits to fledging?

Also is there anything we can be do to giving them the best chance? (we have some bird feeders with seeds/suet pellets but have ben leaving them to it! I'm sure my kids would be more than willing to do some bug hunting!)

Many thanks for any information

KK