My first is response is that you do not need to do anything until the child is resident in the UK. Once the parents are living here and have proof of residence such as an endorsed passport or entry visa then they can apply.
However, if the child holds a full British Citizen passport which is endorsed to show a right of abode in the UK then an application could be accepted even though they are not resident here yet. Similarly if the child is a European Economic Area national then there is unrestricted entry to the UK and they can also apply using an overseas address. However, once the offer of a school place is made then it should be conditional upon receipt of evidence of where they are living.
Just as a reminder, all children of compulsory school age who are resident in the UK have a right of access to education. The only exception might be where the child is only in the UK for a short period such as less than half a term. In these cases it can be reasonable to refuse admission.
Once the child is admitted to your school there will be other issues you will need to address. You don’t mention in your question which country they are living in or if they speak any English. If they don’t and you know the language they do speak you might want to begin making enquiries now to find out if there are any translators available locally. Sometimes if there are a few new families from a particular community there might be someone who can help support during the initial weeks.
If you are receiving a number of new arrivals it might be worthwhile putting together a new arrival policy that includes procedures that all staff can follow. I would recommend that you read the article referred to below ‘A Very Warm Welcome’ to give you some ideas.