Can a bailiff legally question my 13 yr old and hand him paperwork while he on his own?

Yes

The bailiff may not take control of goods where a child is the only person present at the property the goods are located.^[1]

A child is a person under 16 years.^[2]

Bailiffs must withdraw from domestic premises if the only person present is, or appears to be, under the age of 16; they can ask when the debtor will be home.^[3]

Bailiffs must withdraw without making enquiries if the only people present are children who appear to be under the age of 12.^[4]

Bailiffs must not remove anything clearly identifiable as an item belonging to, or for the exclusive use of a person under 16.^[5]

^{[1] Regulation 10(1)(b) of the Taking Control of Goods Regulations 2013
[2] Regulation 2 of the Taking Control of Goods Regulations 2013
[3] Paragraph 72 of the Taking Control of Goods: National Standards 2014, April 2014
[4] Paragraph 73 of the Taking Control of Goods: National Standards 2014, April 2014
[5] Paragraph 64 of the Taking Control of Goods: National Standards 2014, April 2014}