"As a child I always played alone."
Translation:Als Kind habe ich immer allein gespielt.
it sounds kinda unnatural to me as a dutch speaker, and the two languages are very close, but i guess youd need a native speaker to be sure (just to demonstrate how close the two languages are in this case, the sentence in dutch would be: Als kind heb ik altijd alleen gespeeld)
"Alleine" is only used colloquially. http://www.duden.de/rechtschreibung/allein_Adjektiv http://www.duden.de/rechtschreibung/allein_Adverb
Your sentence is perfectly fine. It would be very uncommon to hear it spoken like that, but I can imagine reading it in a autobiography or novel. German speakers prefer the present perfect in speech, probably because it allows you to be more flexible with the intonation (with which word gets stressed).