My friend's granddaughter found this in a rock pool. She said she wants to be a marine biologist when she grows up!
I think it's probably a common shrimp but am not sure. The one next to the child's fishing net is blurred but gives the scale.
Shrimp and prawn are rather widely used terms. The Common shrimp, Crangon crangon, and its ten relatives in the Crangonidae in Britain have tiny rostrums which are easily overlooked, so at a glance you may think they don't have one.
The rostrum is a prow like process that protrudes forwards from the front of the carapace between the eyes. It is well developed with a serrated edge like a saw on the Common prawn, Palaemon serratus, and its relatives in the Palaemonidae, and also in several other groups usually called prawns.
Crangon crangon (shrimp) lives on sand or sandy bottomed pools. P. serratus (prawn) lives in rocky habitats including rock pools.
The broadest point on Crangon is at the very front. The broadest point on Palaemon is set back from the front which narrows on the rostrum.
Your pictures from above show the large rostrum, edge on. You need to look at it from the side to see it fully. To narrow it down to species the serrations have to be examined under magnification. But if not P. serratus, it is certainly one of its relatives and can safely be called a prawn.
I don't know how old your friend's grandaughter is, but I hope this is enough info for you to interpret appropriately to her understanding and to foster her interest. Perhaps she could use her local library and ask for help finding a suitable book that illustrates prawns and shrimps. Or ask a fishmonger who sells unshelled ones if she can have one of each to compare under a magnifier.
Since writing the above, I have seen someone on the "One Show" on TV recording underwater sounds of life in a rock pool. Advanced technical stuff, but unfortunately they constantly referred to a prawn they were recording as a "shrimp". Oh well!