There is a saying, that during Amavasai everyone should partake tiffin at night, but it is not related to tharpanam.
Partaking tiffin by those who perform tharpanam is different from that of everyone partaking tiffin during Amavasai tithi, as partaking cooked rice is prohibited at night on that day based on the saying.
At times, Amavasai starts during the previous night and is present until the next day’s evening; so tharpanam must be done the subsequent day. It means those who do tharpanam must partake tiffin and should not eat anything the previous night.
This should be considered into two parts. Those who do tharpanam must partake tiffin in relation to tharpanam and it is not for others. Tiffin during Amavasai tithi applies to all, be it youngsters or elders, from Vaidika families. But it all depends on the tithi in which it occurs.
Notes:
In many households, those who do tharpanam on Amavasai partake tiffin at night. Elders say children particularly women should partake at least one grain of cooked rice. Two opinions prevail, so it is better to follow the practice of the concerned elders.