Can't help with the newborn and an older one as I've only one DS but some ideas of things to do:
School holiday club or other local club, our football club does holiday club training, we also have local tennis camps
Play dates with friends
Visit to soft play if walkable, older one can play while you sit
Libraries, museums, children's centres, youth clubs, local community group sessions
Cinema trip or film at home
Outside at home: paddling pool, chalk, football and net, hoopla, boules, soft dart board, watering the flowers, sand pit, swing ball, bat and balls, toss and catch ball set - amzn.eu/d/i97ar4N, treasure hunt,
Inside at home: does he have access to a tablet/ console? YouTube kids, films, any apps/ websites he uses for homework, reading books together or him reading to baby, activity books, hot dots activity books amzn.eu/d/7H0hFrT, treasure hunts, playing shops, arts and crafts like play dough where you're not as active
If you're anywhere near Leeds I could probably suggest or particular things!