It appears Islamophobic rabble-rousers on the internet have been distorting the reason behind Prophet Muhammad allowing his grandchildren (Hassan and Husayn, ra) to suck his tongue. 
Sadly, these folk have very little research behind them and even less desire for the truth. They have twisted these actions of genuine devotion and concern for these boys to reflect their debauched minds by accusing the Prophet of homosexuality.
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What is the actual reason behind the tongue-sucking?
The answer lies in Ash-Shifa of Qadi Iyad. The reason behind the tongue-sucking was to quench their thirst and help settle them down. We must remember this was an arid environment (desert) where water was scarce, thus loving parents/guardians did go to such lengths in caring for children:
He gave al-Hasan and al-Husayn his tongue to suck. They had been weeping from thirst and upon this they became quiet. 
This was a genuine act of devotion; had Jesus (p), Gandi, Guru Nanak or Mother Theresa carried out such an act of devotion to children they would have been praised for it. Sadly, this is not always the case for Prophet Muhammad (p) as malicious Islamophobes on the internet have dark agendas they are pursuing assiduously.
Outrageous Islamophobic spin on a genuine act of devotion!
Islamophobes who present Ahadith (narrations) of such a nature in order to promote the idea of homosexuality or child abuse should desist immediately as their spin amounts to nothing more than a fallacious and fanciful attempt at character assassination.
May Allah guide these people. Ameen.
Muslims and Non-Muslims should be alert…
If a hater does present a claim where this occurred between the Prophet (p) and his grandchildren or his daughter (Fatima) please realise there is nothing untoward and the action was a genuine act of devotion that any caring person would undertake for the betterment of the infants. Do not be swayed by such hollow and outrageous distortions.
We must also remember homosexuality is forbidden and sinful in Islam. 
The Prophet’s saliva
Prophet even used to spit in the mouths of suckling children in order to satisfy them until nightfall . Further examples of the blessings within the Prophet’s saliva are given in footnotes , .
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 A couple of narrations Islamophobes erroneously “use” in their efforts to degrade and dehumanize the Prophet (p) and Islam:
Bukhari, 1183. It is related that Abu Hurayra said, "I never sae al-Hasan without my eyes overflowing with tears. That is because the Prophet, may Allah bless him and grant him peace, went out one day and I found him in the mosque. He took my hand and I went along with him. He did not speak to me until we reached the market of Banu Qaynuqa'. He walked around it and looked. Then he left and I left with him until we reached the mosque. He sat down and wrapped himself in his garment. Then he said, 'Where is the little one? Call the little one to me.' Hasan came running and jumped into his lap. Then he put his hand in his beard. Then the Prophet, may Allah bless him and grant him peace, opened his mouth and put his tongue in his mouth. Then he said, O Allah, I love him, so love him and the one who loves him!'" [Al-Adab al-Mufrad Al-Bukhari by Imam BukhariTranslated by: Ustadha Aisha Bewley] From - http://www.sunnipath.com/Library/Hadith/H0003P0046.aspx
Musnad Ahmed Hadith Number 16245, Volume Title: "The Sayings of the Syrians," Chapter Title: "Hadith of Mu’awiya Ibn Abu Sufyan": "I saw the prophet – pbuh – sucking on the tongue or the lips of Al-Hassan son of Ali, may the prayers of Allah be upon him. For no tongue or lips that the prophet sucked on will be tormented (by hell fire)
 Muhammad, Messenger of Allah – Ash Shifa of Qadi Iyad, translated by Aisha Abdarrahman Bewley, Madinah Press, 2004 pg 184
 Imam Dhahabi’s list of enormities (sins), p17.0 he writes, “…There is consensus among both Muslims and the followers of other religions that sodomy is an enormity…” [From the list of enormities presented in Nuh Hamim Keller’s translation of Umdat as Salik, Amana Publications, 2008, p17.0 (page 644)]
 He used to spit into the mouths of suckling children and his saliva would satisfy them until nightfall [page 184 in Ash Sifa of Qadi Iyad, Madinah Press, 2004]
 He spat in a well that was in Anas’ house and there was no water in Madina sweeter than it – Al Bayhaqi [from page 183 in Ash Sifa of Qadi Iyad, Madinah Press, 2004]
 This narration is unclear as to whether the blessings were within the saliva of the Prophet (p) or not but it is of benefit to mention:
In the hadith of Hanash ibn Uqayl we find, “The Messenger of Allah would give me a drink of sawiq (a kind of mash). He would drink first and I would drink last. I always found that it filled me up when I was hungry and quenched me when I was thirsty and was cool when I was parched”
[from page 184 in Ash Shifa of Qadi Iyad, Madinah Press, 2004]