The phrase "pledge your first-born child to" is a sarcastic figure of speech, akin to "sell your soul to". It's saying that someone is asking you to pay too high a price for something, and is often used when you might be getting yourself into a situation that could be hard to get out of later.

In the context of technology, it's often used to say that you'll be tied to a certain platform or system, and switching might be difficult or costly.

For example, say a mobile phone company is offering you a free phone, but you'll need to sign a two-year contract to get it. Some people might think that's too long to commit to one carrier, particularly if you could buy the phone yourself at a reasonable price. In that case, the person who is advising you to reconsider that "deal" might say something like:

Sure, you get a free phone – but, to get it, you have to pledge your first-born child to the carrier.

or, similarly:

Sure, you get a free phone – but you have to sell your soul to get it.

It's very figurative speech to say you might regret something down the road.