The important part of the Tameside quote (kindly provided by Rocky) is this bit:
"A child under fourteen will not be considered in charge of the dog, instead the offence will be deemed to have been caused by the habitual owner who has failed to ensure that a responsible person is in charge of the dog".
What the law actually says is this:
"If a dog defecates at any time on designated land and a person who is in charge of the dog at that time fails to remove the faeces from the land forthwith, that person shall be guilty of an offence . . ."
AND
"For the purposes of this section . . . a person who habitually has a dog in his possession shall be taken to be in charge of the dog at any time unless at that time some other person is in charge of the dog
http://www.legislation.gov.uk/ukpga/1996/20
Tameside Council have taken the view (which I think a court would concur with) that a child under 14 isn't legally 'in charge' of the dog; the owner remains responsible for the animal. Therefore they CAN issue an FPN to the parent (or other owner) of the dog and, if the owner does not accept the FPN and pay it, pursue a criminal prosecution through the courts.
Chris