Hi Nicky,
1. Install “Loco Translate” plugin from wp.org and activate it.
2. Navigate to Dashboard > Loco Transalte > Plugins and click Hotel Booking.
3. Click New Language.
4. Choose “English (United States)” or your current language and press Start translating.
5. Use “filter” field to find all strings with ‘children’.
6. Translate “Children” to “Pets”.
7. Save and check the result.
…and revert back all template php files 😉