Use of vosotros vs ustedes

Question

When you are talking to a young person and mention another person who is older (and vice versa, speaking to an older person and mention a younger person), but refer to the people as you, which verb choice should you use, vosotros or ustedes?

Examples:

John (older person) and you (child) are going to the store tomorrow. How to translate You (both) are going to the store tomorrow (speaking to a child when the older person is not present)?

You (older person) and Maria (child) are going to the store tomorrow. How to translate You (both) are going to the store tomorrow (speaking to the older person when the child is not present)?

Also, does it matter which form is used (vosotros or ustedes) whether or not both people are present (both older person and child)?

Accepted Answer

Hola Missy
This is pretty much a personal choice, there is no formula for this. If you talk directly to an old person and his/her child, you'd just choose the pronoun and conjugation that you think has more weight in that specific case. If you feel you need to be more polite you'd use ustedes. So, any choice would be correct. 
If you use this, though, you need to conjugate the verb formally though:
Usted y su hijo van mañana al mercado. 
You cannot say:
Usted y su hijo vais mañana al mercado. 
Once you use usted to refer to the older person you need to make the verb in the ustedes form,  not the vosotros form. 
As for your last question, if the people are not present, then you will not use vosotros or ustedes, because if they are not present, that subject becomes a "they" as you wouldn't be talking directly to them. 
Ellos van mañana al mercado.
I hope this clarified it.
Saludos
Inma

Answer

Thank you for your replies, Inma. I had quite a few other questions in other lessons over the weekend and I appreciate your quick replies to all of them. Thank you for your thorough replies to everyone who has questions, they are so helpful :)

For this question, I think I'll stick to using ustedes when speaking to both an older person and someone considered as informal, just to be on the safe side, but it's good to know that vosotros can be used too, so when I hear someone using vosotros, I can be aware that they can be speaking to both people in the informal and formal you forms. This really clarified things, thanks !