You have to use the option box align=base
(or tcbox raise base
, which is equivalent) in your \tcbset
.
MWE
\documentclass{book}
\usepackage{tcolorbox}% for framed rounded boxes
\tcbset{colframe=black,colback=white,colupper=black,
fonttitle=\bfseries,nobeforeafter,center title,size=small,box align=base}
\begin{document}
\tcbox{1}, \tcbox{2}, ... ,\tcbox{9} -- numbers
\tcbox{$+$}, \tcbox{$-$}, \tcbox{$\times$}, \tcbox{$\div$} -- operators
\tcbox{$(-)$} -- negative sign
\tcbox{$=$} or \tcbox{\sc{Enter}} -- equal sign
\end{document}
Output

P.S. Don't use \\
to start a new paragraph, but either a blank line or \par
.
EDIT
If you need this behavior only for some of the boxes, assign that property locally instead of globally. This means to remove the option from your \tcbset
and set it for the boxes where it is needed, e.g.
\tcbox[tcbox raise base]{$(-)$}
MWE
\documentclass{book}
\usepackage{tcolorbox}% for framed rounded boxes
\tcbset{colframe=black,colback=white,colupper=black,
fonttitle=\bfseries,nobeforeafter,center title,size=small}
\begin{document}
\tcbox[tcbox raise base]{1}, \tcbox{2}, ... ,\tcbox{9} -- numbers
\tcbox{$+$}, \tcbox{$-$}, \tcbox{$\times$}, \tcbox{$\div$} -- operators
\tcbox[tcbox raise base]{$(-)$} -- negative sign
\tcbox{$=$} or \tcbox{\sc{Enter}} -- equal sign
\end{document}
Output
