I don't want to be the person who earns less than someone else just because they have kids and i don't.
You really don't want to be the person who took more money out of your startup than necessary either though. That's what you're arguing in favour of - a higher burn rate and consequently less time for building your business in order for things to be 'fair'.
If you're in that situation I'd recommend agreeing to track the difference and only pay it in the event of an exit, maybe with a multiple (say, double) to recognise the risk you took. That'd be better than increasing the burn rate just to make things equal.
I honestly don't think this reasoning makes sense. Most people raise investment because it shifts the risk ratio in their favour ("get paid to survive and work on their own idea"), at least in the early stages.
If you're doing this, either: 1) both don't take any salary and use the raised money to build out the business (this is really how it should be), or 2) both take the same salary.
All the rest makes no sense to me and will most likely cause friction, unless one of the founders is already wealthy.
Perhaps someone decided to wait with kids because they want to have more time for the startup and/or more money left over for it.