Your daughter will take some time to write well in her exams, since there is
a change in the medium of instruction. You are wise to think of using the summer vacation to improve her grasp of the English language. There are many
methods: I will mention a few that come to my mind.
Story books are very valuable in this connection. Children will go on reading for the sake of the story, even if they do not understand every word. Books by Enid Blyton, though they are stories of children in another country, have been known to help children to pick up a vocabulary. The local Circulating Library or your neighbours may be able to lend them to your child.
The best method is reading out stories to the child, while she follows the lines, which means you will share the book with the child and let her sit very near you. The Childrens Book Trust has published books, with some coloured illustrations at very low prices and it would be wise to get a dozen books to start with, which are at her present level. As she reads, you can move to slightly advanced books. (Do not make her read her text books for the next academic year, thinking that it will be an advantage). Even Comics speed up childrens reading skill. A few of them may also be added, depending on her preferences.
Similarly, with writing. Leave little notes for her about what she could look forward to in the evening. Use drawings and humour to make the activity enjoyable. Ask her to reply to you in writing. You could device a Silence game for half an hour, where everybody will be quiet and pass out only written messages, on a slate or on paper. If there are children in your area, they could be encouraged to play together. They pick up a lot from each other. TV programmes like Animal Planet have very good visuals and even though they are not addressed to children as such, watching it will familiarise everyone with English usage. Listening to the news in English, for a few minutes at a time, will also reduce the unfamiliarity of its sounds and improve the general knowledge.
Instead of a focus on correcting mistakes, encourage her to say things in English or write them to communicate an idea or feeling. Word Building games, like Antakshari can be fun, where word endings are used for the next word to be made, but they must be aimed at a level where the child can win at least half the time. (I don't mean the one we play with Hindi film songs).
All this takes time and energy! Patience and a sense of humour are great assets and the parents will have to cultivate them. Your daughter will surely catch up in 6 to 12 months.