Suppress ampersand in external text

You can set locally the catcode of & when reading the data. It will not disturb the use of & later, and unlike \detokenize it will not break other commands like non-ascii input in your input:

    \documentclass[10pt]{article}
    \usepackage[T1]{fontenc}
    \usepackage[utf8]{inputenc}
    \usepackage[a4paper]{geometry}

    \catcode`\&=12
    \newcommand{\txtstring}{Father & Sons & Mütter}
    \newcommand{\txtstringtwo}{Father \& Sons & Mütter}
    \newcommand{\txtstringthree}{\detokenize{Father & Sons & Mütter}}
    \catcode`\&=4

    \begin{document}

    \txtstring: OK
    \par
    \txtstringtwo: OK
    \par
    \txtstringthree: Bad

    \begin{tabular}{ll}
    blub & bla
    \end{tabular}
    \end{document}

Here's a solution that just uses \DTLloadrawdb instead of \DTLloaddb:

\documentclass{article}

\usepackage{filecontents}
\usepackage{datatool}

\begin{filecontents*}{test.csv}
CVR,CompanyName,LastName,FirstName,Address,PostCode,City
12 23 45 56,Fox & Sons,Fox,Red,Fowl Street 1,4567,Fox Town
45 56 67 78,Elephant & Daughters,Fant,Eli,Africa Road 1,1259,Mega City
\end{filecontents*}

\DTLloadrawdb{testdata}{test.csv}
\begin{document}
\DTLforeach{testdata}%
{%
    \CV=CVR,%
    \CN=CompanyName,%
    \LN=LastName,%
    \FN=FirstName,%
    \AD=Address,%
    \PC=PostCode,%
    \CT=City%
}%
{%
  \CV, \CN, \LN, \FN, \AD, \PC, \CT\par
}
\end{document}

The ampersands are automatically converted to \& when the data is read from the CSV file. Nothing special needs to be done in the \DTLforeach loop. The above example produces:

Similarly for eight of the other special characters. The only special character not to be mapped is the backslash. You can also add your own mappings (before \DTLloadrawdb). For example:

\DTLrawmap{£}{\pounds}