Original sentence:
Taken aback at the view of a marvellous cat wearing boots and carrying a bag, the guards could not but let him through.
Short, simple correction for a 7-year-old:
The guards were so surprised to see a cat wearing boots and carrying a bag that they let it through.
Why this is better (step by step):
- Taken aback means "very surprised." That phrase is okay but a little old-fashioned. For a child, "were so surprised" is clearer.
- at the view of / by the sight of sounds formal. "To see" or "to see a cat" is simpler and easier to understand.
- could not but let him through is old-fashioned English. Better is "they let it through" or "they let it pass."
- him vs it: we usually say "it" for an animal unless we know the animal is a boy cat. So "it" is safer.
Other simple ways to say it:
- The guards were surprised to see a cat in boots carrying a bag, so they let it pass.
- When the guards saw a cat wearing boots and carrying a bag, they were so surprised they opened the gate for it.
Pick the version you like. The main ideas are: use plain words a 7-year-old knows, keep the sentence short, and use "it" for the cat unless you say the cat is a boy.