The only thing that stands out to me with regard to the concept of the Trinity is from Little Men:
When it ceased at last, and Mrs. Bhaer went to take away the lamp, Demi was gone and Nat fast asleep, lying with his face toward the picture, as if he had already learned to love the Good Man who loved little children, and was a faithful friend to the poor.'
This in reference to Christ seems to imply a rejection of coequality.
In general, the books rarely touch on specific aspects of Protestant beliefs. Major moments I can think of that deal with religious themes are Amy's flirtation with Catholicism in Little Women (which doesn't really touch on Protestant denominations at all), Jo's experience with atheists in the second part of the book, and the Pilgrim's Progress interweaving. Notably, while Mr. March and Meg's son John/Demi are presented as good and wise through their piety and Meg even wants Demi to become a minister, there aren't really theological discussions.
I think there are maybe inferences to be made about how the Marches are portrayed to practise and think about religion, though, whether it's the relative de-emphasis of reading the Bible compared to other texts (although it is frequently quoted), absence of formal church worship (despite Mr. March's profession)--Dan does go to chapel in Jo's Boys, but it's in prison, and solitary or within-family worship is a fixture of the series--or passages like this:
Simple, sincere people seldom speak much of their piety. It shows itself in acts rather than in words, and has more influence than homilies or protestations. Beth could not reason upon or explain the faith that gave her courage and patience to give up life, and cheerfully wait for death. Like a confiding child, she asked no questions, but left everything to God and nature, Father and Mother of us all, feeling sure that they, and they only, could teach and strengthen heart and spirit for this life and the life to come. She did not rebuke Jo with saintly speeches, only loved her better for her passionate affection, and clung more closely to the dear human love, from which our Father never means us to be weaned, but through which He draws us closer to Himself.
(Little Women pt. 2)
While this seems distinctly Transcendentalist in various ways and implies a supremacy of the Lord as the Father alongside the Mother (nature), it's not explicitly Unitarian. This is generally the pattern, where Louisa May Alcott's Transcendentalist beliefs seem to influence her mentions of God, Heaven, or the ability/need for people to strive towards Heaven. However, all the books contain references to the Father in these contexts, and I can't find a single reference to the Son, Christ, etc. except when Amy becomes interested in the Madonna and the Child (which is presented in the Catholic context). Of course, this could also be a function of the political and religious ambiguity of a lot of the text; Alcott would probably be mindful of this in relation to theology.
Returning to 'Father and Mother of us all', this is interesting to me because of the ideas of Christian Science founder Mary Baker Eddy, who corresponded with Alcott's father and (as far as I can tell) Alcott herself. Neither of them were Christian Scientists, but I can imagine the concept of God encompassing maternal traits appealing to Alcott. It's probable however that Alcott is alluding to Mother Nature.