you can buy packages, like
this (action illustrated)
if your just doing slogans i would recommend spending ~$25 on serif's drawplus,
it has some really good text features plus many pre-loaded shapes/distorts/etc.
plus it is quick to learn the basics and you can export to svg for your cutter
go to
fontsquirrel and
dafont (i have the link defaulted to free for commercial use),
download any font that catches your eye
gather your 5 kids round the computer and show them your cool new program for t-shirt design
see which of the 5 it captivates and use it as a bonding/learning opportunity with them
(tell the other children the salt-mine is hiring)