According to the few inormation (No Hadith Number, No Narrtor quoted) i tried to find any support for this "Hadith", what i could say after a certain search:
Well there's no such hadith as far as i can say after a certain research. The only version which seem near to this which is in Musnad al-Imam Ahmad is the Version which says that our Messenger sucked the Tongue of his grand son's narrated by Mu'awiya ibn abi Sufyan (which is quoted in the possible duplicate post).
And the Addition "sucked her tongue" in the Hadith in Sunan abi Dawod is qualified to be da'if but it's not quoting Fatima (May Allah be pleased with her) at all.
A Version excluding many "bad things" could be find in Sunan abi Dawod or al-Adab-Al Mufrad from Imam al Bukhari or Sunan at-Timridi all narrated by Aisha.
It seems that a shi'a Version from bihar al-Anwar بحار الأنوار from al-Majalisi is not far from that one you quoted and a strange addition could be find in فيض القدير في شرح الجامع الصغير Fayd al-Qadir from al-Manawi which seem also near to the quoted hadith and pretends that the Addition "he sucked the tongue" was quoted in Sunan abi Dawod, but the only Hadith which quotes this is the one which i refer to above! And al-Manawi didn't quote a source for the rest of this narration so either he mistaken or mixed between different Narrations: therefore this can't be qualified to be sahih!
But as a conclusion:
I only could find the Hadith you mentioned on websites against Islam pretending either it comes from Musnad Ahmad ibn Hanbal without any other Information (like Narrator, Hadith#) or from some Shia'a books. The only muslim version could only be found in a commentary of a scholar (al-Manawi) without any background or reference, where he also pretends that the addition "he sucked the tongue" is in Sunan abi Dawod, as i could proof this addition is a reason for many scholars to consider the Hadith -which is not quoting Fatima (May Allah be pleased with her) at all- to be weak!
This hadith or better narration isn't from Musnad al-Imam Ahamd ibn Hanbal! No part of it could be found in any sahih book therefore this hadith or narration (must have/ or) has been fabricated
And Allah knows best!