your kids are ata great age for a move to Spain and kids will adpat and learn the language very comfortably they will need help from you and private lessons for the whole family is recommended for the first 6 months or so and get the private tutor to work with the school. Parenst should focus on the grammar so they can help with school homework etc.
Spanish schools are very good but you have to research just as you woud in the UK
Pre school for 3 to 6 yrs is free and optional but recommended as they will help with the language
Under 3 you would have to pay
Health care may be an issue for you as you will not get covered and as part of your "residency" you will have to show you have adeqaute cover and money to live on etc.
Alternatively you could stay under the radar (not recommended) and pay private schools which is always an option
Great life style for kids in Spain as they are welcome everywhere with parents
Many activities available for kids
Good luck