Ultimately, you know your kiddo(s) best.
The junior books are geared towards kids in the 3-6 ish age bracket, and the senior books are aimed at kids in the 7-10 ish age bracket, but these are really just a guide, based on ‘average’ abilities and interests. We've had kids aged 11 and 12 love the senior travel diary too.
In terms of academic skill-level, the junior book primarily focuses on skills taught at pre-school and an early primary level. Things like pre-writing development, letters and numbers, colours, shape recognition and simple problem-solving.
The senior book focuses on skills developed in middle to later primary school. Things like writing, sight words, basic math's, general problem-solving and geography. As well as confidence building exercises and comprehension.
In each book the cultural content (the part that teaches your child about your chosen holiday destination) and the video content (the part that teaches your child the local language) is the same.