Can a ban on surrogacy be justified by the welfare or dignity of the child?

Not in principle. The feared harms of split motherhood lack solid empirical evidence, and the dignity objection (the child as "goods") fails if the surrogate is paid for the service of carrying, not for the child. Besides, the child's alternative would be never to have existed, which speaks more for surrogacy than against it.

Welfare arguments: Germany bans egg donation and surrogacy on the presumption that split motherhood harms the child. Supporting reasons: (1) a negative attitude of the surrogate might disturb foetal development, (2) conflicts between the birth mother and the social mother after birth, (3) the child's search for identity could suffer. But there is no conclusive empirical evidence for these, so a ban in principle cannot be justified on welfare grounds.

Dignity argument: the child's human dignity is violated because it is treated as merchandise. Replies:

• It contradicts the fact that altruistic surrogacy exists.
• Kant's formula of humanity forbids treating others merely as a means and never also as an end in themselves. The surrogate is paid for her service of carrying the child, not for the child itself.
• Parental rights must not be confused with property rights.
• Is a right of the child violated at all? Its alternative would have been never to exist. Hence this is more a reason for surrogacy than against it.

Go deeper:

• Wikipedia: Non-identity problem — the philosophical puzzle behind "the alternative is never existing".
• Wikipedia: Categorical imperative — Kant's "never merely as a means" formula.