Can a bailiff take my 11 year old child's computer?

No

Bailiffs may not take control of goods of any debtor that is under the age of 16.^[1][2]

If anyone, including the debtor, uses the computer for their work or school, it is exempt goods,^[3] and the owner may make a claim to exempt goods.^[4]

^{[1] Regulation 2 of the Taking Control of Goods (Fees) Regulations 2014
[2] Regulation 10(1)(a) of the Taking Control of Goods (Fees) Regulations 2014
[3] Regulation 4(1)(a) of the Taking Control of Goods Regulations 2013
[4] See Making a claim to exempt goods}