I wrote 5 sentences for you to check. Thank you.

1) In their backpacks they have packed extra clothes, food and water for the two-day competition. I can see the finishing line in the background (on the top left-hand side??)
2) The first boy on the left is wearing a two-colour flowery short-sleeved shirt with a yellow T-shirt under it, black tracksuit bottoms with a white stripe on each side and a white-and-red bandanna on his forehead.
3) The second boy on the left is wearing the same yellow T-shirt and khaki bottoms with pockets on both sides, black knee high gaiters (?) and hiking boots.
4) The boy on the very left is wearing a checked shirt with a yellow T-shirt over it, khaki trousers and ankle-high boots. He is holding a walking stick in each hand (?).
5)They are all wearing a backpack and a sleeping bag on their back and a yellow T-shirt with the competition’s sponsors on it.

2. bandana

3. knee-high ...But the SAME yellow T-shirt? Perhaps "also wearing a yellow, etc.?
4. checked or checkered

Sra

Here are the checked sentences:

1) In their backpacks, they have packed extra clothes, food, and water for the two-day competition. I can see the finishing line in the background (on the top left-hand side??)
Explanation: The sentence is grammatically correct, but there is a question mark after "on the top left-hand side." If you are unsure about the exact location of the finishing line, it is appropriate to include a question mark.

2) The first boy on the left is wearing a two-color flowery short-sleeved shirt with a yellow T-shirt under it, black tracksuit bottoms with a white stripe on each side, and a white-and-red bandanna on his forehead.
Explanation: The sentence is grammatically correct and provides a detailed description of the clothing of the first boy on the left.

3) The second boy on the left is wearing the same yellow T-shirt and khaki bottoms with pockets on both sides, black knee-high gaiters (?) and hiking boots.
Explanation: The sentence is grammatically correct. However, there is a question mark after "black knee-high gaiters." If you are not certain about the correct term for the leg coverings, it is appropriate to include a question mark.

4) The boy on the very left is wearing a checked shirt with a yellow T-shirt over it, khaki trousers, and ankle-high boots. He is holding a walking stick in each hand (?).
Explanation: The sentence is grammatically correct, but there is a question mark after "He is holding a walking stick in each hand." If you are unsure about the accuracy of this detail, it is appropriate to include a question mark.

5) They are all wearing a backpack and a sleeping bag on their back and a yellow T-shirt with the competition's sponsors on it.
Explanation: The sentence is grammatically correct and effectively describes what each person in the group is wearing for the competition.