Toys R Us in Cambridge

I bought a child bike (7-9 years old) on 7th Feb, 2015 from Toys R Us, Cambridge. The bike was assembled by the shop. After they said everything was fine, I delivered the bike home. Then my daughter tried it, and it is too heavy to ride. so I had to take the bike back to the shop on 14th Feb. I told the manager the bike was too heavy to ride, but he checked it and said everything was fine as this is a brand new bike. Yes, I bought a brand new bike, but it can not be ride, and even I took it back to the shop, they still told me everything is fine! The manager told me the new bike needed to be rubbed to make it work smoothly, so I have to deliver the bike home again. My daughter tried the bike again, but nothing changed. It is just too heavy to ride! Then I phoned the manager, I told the manager the bike is too difficult to ride, but he said "it is easy to ride!". I was speechless. He rides the bike or a child rides the bike?! He told me I can deliver the bike to the shop at any time, and he can refund it straight away. So I took the bike to the shop again on 21st Feb, but the other manager said he was not there, and refuse to refund. I delivered the bike home twice and it never work! I am really frustrated and annoyed by the service provided by that shop.