When I bought first bikes for my boys, I didn't want them too babyish. It's actually quite a challenge to find a small bike that isn't Paw Patrol or Peppa Pig.
Argos sell Huffy Bikes, and have them half price just now. These are much more grown up looking and are VERY highly rated.
Ride on Toys
ARGOS DISCOUNT CODE:
Argos: 10% discount on selected Hisense fridge freezers