All Praise to Allah Subhanahu wa Taala and blessings of Him be on Prophet Muhammad peace be upon him His family and companions,
Narrated Ibn Abbas (RA):
That the mother of Saad bin Ubada the brother of Bani Saida died in
Saad's absence, so he came to the Prophet saying, "O Allah's Messenger
(ﷺ)! My mother died in my absence, will it benefit her if I give in
charity on her behalf?" The Prophet (ﷺ) said, "Yes." Sa`d said, "I
take you as my witness that I give my garden Al-Makhraf in charity on
Narrated `Aisha: A man said to the Prophet, "My mother died suddenly,
and I think that if she could speak, she would have given in charity.
May I give in charity on her behalf?" He said, "Yes! Give in charity
on her behalf."(Bukhari)
It is fine in both ways, you could tell them if you feel or if you have just made an intention in your heart then this way is also fine.
"Verily actions are by intention." [Al-Bukhari and Muslim]
It is not like that rewards have to be equally distributed or rewards would be calculated. Our Lord Allah Swt is As Samad and rahmah of our lord finds an excuses to bless us and distribute rewards so everyone would be rewarded uncountable In Sha Allah.
And Allah and his Messenger knows best.