I have changed the derailleur on my daughters back, bought like for like.
When I try to pedal the derailleur, it flicks backwards and the bike won’t pedal forward, it remains stuck.
Any advice would be greatly appreciated.
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I think we can rule out the chain being too long. The OP replaced the derailleur but does not state the chain was replaced. Also, that does not look like a clean and shiny new chain.
The chain is threaded through the cage correctly, it's just that the derailleur had rotated 90 degrees rearward so that the cage is sitting behind the sprockets rather then underneath them. If the derailleur was rotated back it's possible to see that the chain would run in front of the top jockey wheel and behind the lower one.
Here is my theory: the cage is not tensioning the chain, it should be trying to rotate clockwise under spring tension but is not. I think the cage was somehow rotated clockwise before the chain was threaded through it, unloading its spring.
To fix, remove the chain, rotate the cage anti clockwise so that its spring is wound up, rethread the chain through the cage and rejoin. The chain should be under quite a bit of tension when you join it.
Looking at it again, I think all that's happened is the mounting bolt was done up with the derailleur in the wrong place. It's also possible the bolt is loose, hence "flicking back" rather than stuck back.
I reckon the cure is to loosen the mounting bolt, correctly position the derailleur, with the B screw pressing against the hanger, then tighten the mounting bolt again. It should be properly tight - there's a chance it was only done up lightly and worked looser in the first rides, meaning it would be likely to fall off.