Well, if your business was in Canada or Europe, you'd have to figure it out. And as a parent who has taken advantage of this, it's worthwhile due to the positive effect it has on the children.

And if that senior dev just left for a better job?

Same thing would've probably happened, but that's not a risk you can really mitigate against. I believe he even had some ownership of the company, so he probably lost out on a bit too.

With all the possible factors: leaving for a better job, falling ill, having a family member fall ill, getting hit by bus, getting poached by a competitor, or taking parental leave -- that last one is hardly even the worst. If you're worried about that, you're not worried enough about real issues.