Managing with the languagesHow quickly do the children learn?
For all the prepartion you make, children will pretty much get thrown in at the deep end. We were lucky with a supportive school and friendly children, but it will take around 6 months to get past the basics.
The first three months were the hardest. Even with preparation, the types of language the children will learn in the UK, will be very different from that they first encounter at school, and for young children language learning in the UK is a game, not serious.
We were lucky that the children's school has three teachers of English and provides classes for non-Catalan speakers, but even so do not expect instant language skills.
It took about 6 months for the children to start to be able to interact linguistically with other children (simple conversations) and to be able to follow the teacher understanding 40-50% of what was being asked.
After about 9-12 months, we had our first revelation moment, when our eldest said he'd put his hand up and answered his first question in class. Very proud of it he was too.
Around 15-18 months the children started to be fluent enough that they could start to achieve similar standards in controls (tests) to the level we would expect in England, but still lacked some fluency in both speaking and writing and still had English accents to a certain extent.
After about 2 years they have reached a level where they can hold fluent conversations with friends and adults and have fluency in their writing, albeit still with some basic mistakes. They are also now more able to separate out Spanish and Catalan.
They will still struggle compared to native speakers because they will only run into words and terms at school, whereas native speakers will be introduced to ideas by their parents - for instance we'd describe a river delta or a fossil in English, whereas at school they need to know and understand this in Catalan.
Traps
A few little traps for the unwary. The first language they learnt were English swear words - the other children thought they were cool for being English and so taught them all the words they wouldn't know or be able to use in an English school.
After English swear words, they learnt Spanish ones (tacos). As a parent you will need to know what these are so you can gently guide your daughter away from shouting 'Joder' at the top of her voice in your local supermarket.
Don't expect instant language and do expect to have to support them a lot through homework initially. They still need help now from time to time, particularly where the question is phrased oddly, or where they've had a question dictated to them and don't know if they wrote it down correctly.
