An idiom is a common saying which really makes no sense if you take it literally. Unfortunately, we assume children know them as they are very well-known by adults. However, children hear things and don't ask about their meaning or else they assume them to be literal and they simply don't try to deconstruct them.
Like other examples, the better the level of your child's reading, the better their chance of succeeding in such questions. Also, we must encourage children to question anything that they aren't sure about and be prepared to answer those queries regardless of what we are doing, or else they will give up and not bother.
Here are some examples of the kind of questions that some schools may set:
Complete the following sentences, which contain idioms, with the most suitable word:
1. The issue had become a bit of a hot ______
2. I decided there was no point crying over spilt ______
The lists of idioms that you can find online will be invaluable here. Check them out and encourage your child to point out the ones that they already know - this way, they will get the idea of how they work and they will naturally be inquisitive about the ones they don't know. Do it the other way round, and you can easily switch off their interest - always encourage them to show off their knowledge rather than highlight what they don't yet know.
Common sense is hard to apply for idioms so if they don't recognise the answers, it's tough to guess. Often the first part of the sentence will bear no relevance to the word that is missing. A contentious issue is a 'hot potato' - there are no real clues, you simply have to be familiar with the phrase. It would help to visualise the object and see whether it exists; should it not, there's little chance of it being right. A 'hot plant' or a 'hot ball' don't really exist, whereas a 'hot potato' is reasonable.
The second answer, of course, is 'spilt milk' and the only answer that could be ruled out from the original list is 'feelings', as they can't be spilt. We simply have to encourage children to listen to language and read carefully.
Look at the following words. They all have a similar root (or stem) word.
Using your knowledge of the words, explain what you think the root word, 'port', means.
This is a very good test to see whether your child thinks beyond the obvious. There are a lot of words in the English language which are derived from Latin or Greek and the new words that get added have often got origins that are thought-out with relation to the ancient languages. In this case, we have to hope that your child is able to work out what certain words have in common as they will certainly not be expected to have any sort of working knowledge of Latin!
All the words in the question have a connection. If your child is uncertain, get them to visualise what is going on as the action is being carried out. If they don't know what 'deport' means, get them to picture what is happening when someone 'transports' something. They should have heard of 'export' or 'import' and can look for connections between their ideas of what the words entail and what 'transport' means. Someone is taking something from one place to another when they 'transport' it. Similarly, 'import' and 'export' involve taking things from one place to another, whether it is into or out of the country. 'Deport' involves taking someone away from a place; the answer will therefore be a word that connects all of these.
Of the possible answers, 'bring' would make some sense but it goes AGAINST 'export' and 'deport'. Conversely, 'out' would make sense for those two but not 'import' or 'transport'. Although 'port' could be a bit like a 'station', that makes no sense with the meaning of the whole words. The answer has to be 'carry' as we 'carry across' when we 'transport' something and then 'carry away' when we 'deport', etc. Learning a few simple prefixes is a useful idea to help make sense of things.