Earlier quoted context omitted.
(Some) people will design the new human for the benefit of themselves (i.e. the designer) rather than the benefit of the new human. A few ideas that come to mind are having children with ridiculously amped loyalty, so I can condition them during their early years and then put them to work making money and simply use them as my retirement fund. I could also ensure they're suitable organ donors, and make use of their o…
There's already a demographic that'd do that sort of thing - see reddit.com/r/raisedbynarcissists for horrifying examples. Your concern is dead on, and we'd need some serious progress on youth rights for me to be more comfortable with unrestricted designer babies.
Interestingly, I think the current systems in most of the developed would would already handle this somewhat adequately, insofar as "designer babies" would probably not be treated as genetic children of their "gene base" parents (i.e. something they have an intrinsic right to take home from the hospital), but probably rather would be considered "genetic orphans" (like babies dropped off at a hospital with no known parents) who are then adopted by the "gene base" parents. As such, just like any other adopted children, the parents would have to go through harsh screenings to determine eligibility, and then after "adoption" would be heavily monitored by governmental child social-welfare departments (e.g. Child Protective Services in the US.)