Bike derailleur, keeps flicking back when I pedal I have changed the derailleur on my daughters back, bought like for like.

Now fitted.

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.
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.