The OP's question wasn't whether the Saningers have the right to sue, but whether they should be allowed to raise the child.
They are raising her, and can continue unless it's judged that the parents' actions put the child in danger. Yes, they're causing harm to their child and arguably have demonstrated they do not have the child's best interests at heart. If someone - an interested party, or the authorities - made a case to remove the child it would be up to the courts to decide what's best (least bad) for the child. The Saningers claim that they HAVE to sue to recoup the money they have to spend on their "extra" child. Presumably, the money they would gain from the lawsuit (if successful) will benefit all of their children. Win or lose, harm has been done by bringing the suit. Unless the child is removed from the home and given a new identity she will suffer, but the process of removing her and giving her a new identity would also be a hardship for her.
RE the current case - no existing process is 100% effective to predict the sex of an individual embryo pre-implantation. For this reason, clinics don't generally offer a guarantee that the foetus initially implanted will be a particular sex, although they may offer after-the-fact sex selection ("selective abortion") as a backup to PGD/PGS in states where that's legal and covered by the contract.
It appears in this case that the written contract did NOT guarantee the sex of the implanted foetus. The Saningers say that (1) they were verbally assured the clinic could guarantee the sex of the implanted foetus (2) the reputation of the clinic is such that they believed the claims without thoroughly investigating and (3) the clinic knew the Saningers only wanted boys. There's a good case for breach of (commercial) contract here, depending on available documentation.
Not so sure about malpractice. In a California context, there have been successful claims where it's judged that a vendor or medical facility's actions "confounded efforts to have or avoid a child born with particular traits". But it's previously been in cases where there was demonstrable harm done: for example, a couple screening out male foetusus to avoid a congenital birth defect has a male child whose health is severely compromised by that condition (Bergero vs University of Southern California).
Usually you have to prove actual damage to life, health, or property - hence the Saningers' claim that having one healthy baby of each sex rather than two healthy boys forces the couple to repeat the expense of IVF/surrogacy unnecessarily and to take on the cost of raising three children instead of two. So we'll see if the California courts rule that the birth of a healthy female baby constitutes actual harm - and expect it to be appealed whatever they decide.