Rome Termini to Venice S. Lucia - how to add a seat for 2 Y child ?

Hi,

I am trying to reserve seats for my 2Y child for the route  18:35 Rome Termini  --->22:59 Venice S. Lucia from https://www.raileurope.com/  . So as per instruction in website I am putting age as 6. At time of selecting seats and payment, website asking for my child date of birth. I am putting actual date of birth. App validating the age entered (6) and age as per date of birth mentioned (2) and showing error - “Passenger's date of birth doesn't match age: <name> will be 2 at time of travel”.

Please suggest how can i book. Since its 4.5 hr of journey, i wanted to have seat for my child.

DOB is a mandatory field on the form which i cannot skip.