Kids under 4 can travel for free on Eurostar, provided they can sit on an adult's lap. Here's how it works:

Your child will need to sit on your lap throughout the whole journey.
If you want them to have their own seat you'll need to buy a child ticket (see below).
I you are travelling with more than one child under 4, they'll each need to sit on an adult's lap.
If you are travelling alone with two or more kids under 4, you'll need to take one on your lap and buy child tickets for the others.
Remember, kids of any age need their own passport to travel.

Can I travel with my child under 4 years old?

Travel with children