Can a bailiff give a notice letter to a child to pass to a parent?

Yes

When attending a property, bailiffs may not take control of goods when the debtor is a child under 16.^[1][2]

If a vulnerable person, ^[3] or child under 12 is alone in the property, the bailiff must quietly leave the property without making enquiries.^[4]

Bailiffs should be trained to be aware that a person's vulnerability might not be physically obvious.^[5]

^{[1] Regulation 2 of the Taking Control of Goods Regulations 2013
[2] Regulation 10(1)(a) of the Taking Control of Goods Regulations 2013
[3] See Bailiffs and enforcement against vulnerable people
[4] Paragraph 73 of the Taking Control of Goods National Standards, published by the Ministry of Justice
[5] Paragraph 76 of the Taking Control of Goods: National Standards, April 2014}