I was just reading the gentle book thread, and it started me thinking about this book. I can't remember the name of it and it's driving me mad.
It was aimed at ages 8-10.
I think it was part of a series. It was similar to 'my naughty little sister' but maybe for a bit older, and I think it was more recently written - I read it in the mid to late nineties.
The protagonist is a girl - slightly neglected possibly due to her mum having lots of kids. Possibly a bit of a tomboy.
I think in the first book she starts at a new school (or maybe joins brownies...see later) , behaves atrociously, gets bullied (but I seem to recall this as being presented as her deserving it 
). Great big redemption arc and hurrah everyone's friends.
Second book is slightly clearer...
She is now happily in brownies. However the big brownie trip (to Paris?) is coming up. One of her numerous (?) little siblings comes down with chicken pox, so her mum says 'my children always get chicken pox two weeks apart.' Which coincides with the Brownie trip to Paris so she can't go.
Her best friend's mum (best friend was, I think, her big enemy in book 1) says not to worry, she can come and stay with her and best friend for the two weeks prior to the trip (yup, lots of misunderstanding about chicken pox here) so that she won't catch it and will be able to go.
Possibly more adventures in two week run up. I seem to remember them getting the ferry to calais.
I've googled, but can't find anything. Not Enid Blyton.