I will summarise what I have learnt from Nicolas Thiery:

If M is a monoid then the monoid algebra is constructing using 
Monoid().Algebra() by, say, M.algebra(QQ).
For groups the construction GroupAlgebra is being replaced by a similar 
G.algebra().
This is part of an ongoing program to replace functions using FormalSums by 
functions using CombinatorialFreeModule()

The directed graph C.category_graph() can be drawn using graphviz. 
Specifically

    sage: C = Monoids().Algebras(QQ)
    sage: G = C.category_graph()
    sage: G.set_latex_options(format="dot2tex")
    sage: view(G, viewer="pdf", tightpage=True)

I have not personally tested this as I would have to install the optional 
packages graphviz and dot2tex

A group algebra is constructed as a Hopf algebra but a monoid algebra is not 
constructed as a bialgebra
although it would be straightforward for it to do so.

I am still confused about categories and parents and am unclear on how to 
implement a new parent.

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