Posted on 20 May 2014
Have a look at
http://www.festivalkidz.com/tag/best-family-festival/ and under 'Best festivals for toddlers 2014' click on 'more' to bring up dates. These are aimed at children up to 5yrs old.
Admittedly some have already gone or are very soon, but it may give you a few ideas, maybe some for next year.
Have you any other specific interests/festivals in mind that his children may enjoy? This may help others to add to this.
HTH