There is no way to add a child's pass. What you need to do is to exchange one of the adult passes for an adult pass and a child's pass. The fee for an exchange of passes is 15 EUR. 

https://www.eurail.com/en/eurail-passes/ordering-info/refund-exchange-policy

What a nightmare. Does she actually need a pass or can we leave it as long as we choose not to buy seat reservations for her? There were literally no options to add a child when we booked so I am confused about where we were supposed to do it. Is there a way to contact eurail? 

You don't need a child pass if you don't want to do seat reservations for her.

https://www.eurail.com/en/eurail-passes/deals/eurail-pass-discounts/family-discounts

That's the ordering page (you just need to add a free child pass; it's that easy):

You either pay 15 EUR, which isn't much compared to what your trip to Europe will cost, or your daughter needs to sit on your lap for the whole trip.

Thanks Hektor, I saw that but she will be 1 and 2 years old on the trip so I didn’t think to add it as obviously thought it didn’t apply to her? Also, we have already ordered and paid so was wondering how I can do it now? 

Thanks Anna, I appreciate that and obviously will do it, but just isn’t very clear for children aged 0-4?? Seems like an obvious mistake for families to make 

For ages 4-11 you do need the free child pass, for ages 0-3 just if you'd like to do reservations for a child. 

But it's clearly explained in my first link, too (just at the beginning).

Ok thanks both for your help!! 

I agree that it not is clear on this page that you need a pass for children under 4 if you want to make reservations.