London with 3 Young Children

Hi, bit of a random one, but hoping for some help. My fiancé is running the London marathon at the end of April and we are travelling as a family from our home in rural Wales for a long weekend in the city. Our boys are aged 7, 2 and 1. We have never been to London (or any big city, for that matter 🙈😆) before so are completely out of our comfort zone! On marathon day, I will obviously be completely alone with the kids and need somewhere safe, and fun to take them while daddy runs 🏃🏻‍♂️ We’re staying in the Docks (?!) area, if that makes any sense 🙈 If anyone has any suggestions then I’d be very grateful
thanks