The allegation of the non Ahmadi Muslims is that Hadhrat Mirza Ghulam Ahmadas has written that the Holy Prophet Muhammadsaw had 11 sons. This is a false allegation and in reality a allegation against the scholars of the non Ahmadi Muslims.
In reality, whenever heas spoke about this, he was referring to what the historians have said about the Life of our Beloved Prophet Muhammadsaw. Heas has clearly written that “Historians know” right before mentioning this statement. It is mentioned in his book called Chashm-e-Marifat. In Tajalliyat-e-Illahiyyah the same is being referred to . despite the Promised Messiahas not saying the word “historians”. Below you will find the passage of Chasm-e-Marifat where it is clearly mentioned that the Messiahas is referring to the historians, and after that I will quote some seerat books of the non Ahmadi Muslims also claiming that the Prophet Muhammadsaw had 11 sons, which proves the Promised Messiahas truthful once again.
In Seerat Halbiyya, 11 sons are also mentioned:
The second reference comes from the Seerat book of Hadhrat Shah Waliullah Dehvirh:
Lastly, the same is mentioned in Bihar ul Anwar:
These references further make it clear that this allegation is just another lie of the anti Ahmadis which in reality is an accusation against their mujadids and scholars.
I end this answer with a Youtube link of a channel that has over 27 million views and is watched by millions of Muslims. They themselves have also admit there are many different narrations saying different things, and that according to them, the Hoy Prophetsaw had 7 sons: https://www.youtube.com/watch?v=dDuiaLB88f0&feature=youtu.be