What's the correct meaning here?

If the only reason you want to have children is so that they can take care of you in your retirement, having children isn't worthwhile. That's what they're saying.

"in addition to". It's saying you shouldn't have children if you are only doing so because you dream of them taking care of you when you are old. Presumably they are discussing someone who had children & did not take care of them as they grew up, to be surprised when the children returned the favour.

The sentence is implying that many people have children so they have someone to take care of them when they are older. So, having children is worthwhile if you are doing it for reasons besides just having someone to care for you in old age.

Kind of both and neither. The logic is sort of... 1. Some people only have kids so that there will be someone who feels obligated to care for them when they get old; the children are their "retirement plan." 2. (1) is not a very good reason to have children, actually. It can be damaging for the children and create a toxic parent/child relationship. 3. There are reasons other than (1) to have kids, though. Some reasons are quite healthy and promote the well-being of both the parent and the child. 4. If you want kids for a reason other than (1), it's good to have kids. If you have kids just because of (1), you might regret it.

It seems like perhaps the intent was "There are good reasons to have children beyond just having someone to take care of you in your old age." But I'm guessing here. It's a strange sentence, especially without any context.

I'd interpret it as "not only a retirement plan"; i.e., there are reasons other than r.p., but there needn't necessarily be r.p. as part of the reasons.

“Beyond” is the different meaning from “in addition” or “instead of”. i.e. you want to have children for better reasons outside of them solely being your retirement plan.

The phrase implies that "having children" = "retirement plan" and that you should have additional reasons for having children in order for it to be worthwhile. It's a way of saying, "don't have kids the only reason is for them to take care of you in retirement."

Beyond in this case means in addition to.

It means that if your only reason for having children is to help you when you're old, you shouldn't have children. The speaker means that if you intend to have children, you should also be interested in caring for, teaching, nurturing, and preparing them to be their own individual people. It is expensive raising children. People who only want children to help them when they're old should save their money for elder care.

In this type of sentence, beyond usually means “in addition to.”

It could mean either. I imagine it meaning "In addition to".