Without reading all the terms of your IVA it is not possible to say with certainty what would happen if you fail it. I doubt whether there is anything in it which means you HAVE to pay them your redundancy money, but there will -as Ethel says - very likely be something saying in effect that if you don't pay any lump sum you receive the IVA has failed.
What happens if it does fail is that your debt position is back to square 1 - you still have all the debts you had when you started the IVA (minus any money the creditors actually received), with interest etc. added on to them. Also, some IVAs include a clause requiring the IP to bankrupt you if the IVA fails.
If you are bankrupted (or go bankrupt yourself) then - again as Ethel says - you will lose the redundancy money, although not necessarily all of it as you might have an argument for keeping some of it to live on if you are pregnant & unable to work. This would be for the OR to decide.
If you fail the IVA & do not become bankrupt you will have to make your own arrangements with your creditors.
If you are going to have to claim means tested benefits (income support; housing benefit etc.) which is possible if you have a baby then you need to be careful to make absolutely certain that any payments you make out of the redundancy money are for debts payment of which has already been demanded, otherwise the DWP may treat you as still having the capital & reduce your benefits. I imagine you will be able to prove this as I assume all the debts will have been demanded before they went into the IVA, but you do need to be certain about it.
If you need to talk all this through it would be worth a visit to your loal CAB.