So what if you have 100 child chains. Some are the equivalent of rimbit, whereas others have $100 million businesses built on them. Will any of the tokens be exchangeable 1:1 with fNXT?!
No, there will be no such peg, even for NXT itself. Only at the hard fork block we start at 1:1 distribution, but when a ChildcoinBlock creator creates a block, he can offer more, or less, fNXT, to the forger, if the market value of the tokens included as fees has increased or decreased.
A business like Rimbit can decide to subsidize their transactions and keep paying fees in fNXT even if the real market value of the Rimbit included is not equivalent, and this should be fine, because the forgers do get their fNXT. But such business must have a way of making money somehow, to be able to buy those fNXTs, if they want to continue this indefinitely.
How a new child chain will be bootstrapped, what starting point for the value of its token to use, I don't know yet. It could be that first just the token is created and trading to/from fNXT allowed, to establish some baseline market value. And if this is manipulated by the chain creator, it is still ok, they cannot continue the manipulation without running out of funds at some point. The initial cost of creating a child chain should be very high, to protect against abuse.