Translation:We assume that they have children.
The simplified answer: because dass sie Kinder haben is too long to be squished into the middle as Wir nehmen dass sie Kinder haben an, and that would sound confusing even to Germans.
The technical answer: an doesn't actually go to the end of the sentence; it belongs in the "right bracket" position (rechte Klammer) which comes after the "mid-field" position (Mittelfeld) which contains the largest part of most sentences but before the optional "final field" (Nachfeld).
The Nachfeld is very often empty and so the right bracket position is often also the end of the sentence, but dass clauses that are the subject or the object of a verb pretty much have to go there, rather than in the middle -- and then things that go "at the end" (really: in the right bracket) such as separable prefixes or participles end up in front of that clause.
See also https://de.wikipedia.org/wiki/Feldermodell_des_deutschen_Satzes for more than you ever wanted to know about the field model of German sentences.
kinder and kindern are not even German words.
Kinder (capitalised -- it's a noun!) is the plural of Kind, i.e. "children"
Kindern is the dative case form of the plural -- in all other cases, the word is Kinder.
Here, you need the accusative case, as it's the direct object of the verb haben.