Hi Mary,
There are sites out there which can help you with this, for example:
www.mylanguageexchange.com
They do charge a fee but I don't know how much.
Otherwise an idea might be to contact a good school in the UK and propose something directly to the Head of the Language Department.
That takes a bit of time but in the days of internet and email it should be feasible - better idea - maybe there is a keen parent out there who has the time and inclination to source this for you.
Public schools (paid education) often have more motivated parents who would be keen for their kids to participate - of course there are PLENTY of good state schools too. You want a state school in a good area or a public school for best results.
Finally, here is an in-house idea that you might like - totally free!
https://teachingenglishgames.com/pen-pals/
Kind regards
Shelley
Games, stories, songs for teaching English - especially children from ages 3 to 12: