How can I get my children involved in chess tournaments (UK)?

There are three levels for kids to get involved in chess tournaments:
1. Getting into the school chess club:
Some schools have an active chess club that hosts internal tournaments and also send teams to participate in inter-school tournaments. This depends highly on the teacher in charge and whether they take initiative to promote a chess culture in the school.
2. Joining a local chess club:
If the school does not have an active chess club, it is time to look for a local chess club instead. A simple Google search for "[Your Area] Chess Club" should suffice. Find out the weekly club meetings and the contact of the person managing the club.
Some follow-up questions you might like to ask during the meeting include:
-Are there local tournaments held for younger players to participate in?
-What events are held suitable for your child's age group?
-Is there a junior league?
3.Finding tournaments on the English Chess Federation website:
https://www.englishchess.org.uk/event-calendar/
On the link above, you will find a range of tournaments that take place all over the UK. This includes Open/Junior tournaments and face-to-face/online tournaments as well. You can filter the list and select the ones that best fit your needs.
All the best! :)