Mum swears by eucalyptus oil. Maybe put the boots outside to get the chewing gum cold & scrape off as much as you can (gently because you don't want to damage the leather), then use eucalyptus oil with a cotton bud or ball. Probably best to sit on the deck for this one, unless the boys are wheezy and do it in the loungeroom.

Do you have any leather cleaner for your couches?

All else fails - take it to an old-school bootmaker/repairer, they may be able to clean it off for you.