Considering that enlargement of the spleen is possible even in non-thalassemia individuals (like from catching a virus like mononucleosis), it is not entirely unusual - in my opinion - that even a very young thal patient can have enlargement of the spleen - especially in response to illness and febrile illness especially.
Spleen enlargement seems to come about in response to how hard it has to work to do its job, so it is logical to suggest that anything that stresses it out more has the potential to make the spleen enlarge in response.
I have had an enlarged spleen since I was a very young child despite having an intermediate form of thalassemia and every time I have been sick with something more significant than a cold, my spleen tends to enlarge a bit more until my body has finished recuperating. After the illness is over with, the extra enlargement usually calms down.
My cousin is a doctor of Naturopathy and has before mentioned that these herbs can help alleviate an enlarged spleen and the symptoms related to it in some patients:
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Polymnia UvedaliaAn old (and I mean old) doctor's report on using this herb-
Carduus Marianus - more commonly known as St. Mary's Thistle
It is usually used more to help liver issues - especially hepatitis, but it supposedly works in a similar manner to help aid the spleen and reduce enlargement or potential for enlargement.
Both of these herbs can be researched online, but as with all things, I would consult with someone who knows these herbs better and what they can do and how they interact with the body.
Best of luck!