As Minnie and Mickey are the flagship characters, you can get all sorts of merchandise.
Harringtons on Main Street and the glass shop sell blown glass and crystal objects.
The Emporium http://www.disneylandparis.ie/shops/disneyland-park/emporium/
in Liberty Arcade on Main Street is a general store which sells Mickey Ears, costumes, toys, plushes, houseware such as mugs and glasses, clothes and accessories.
Disney & Co http://www.disneylandparis.ie/shops/disneyland-park/disney-and-co/
in Liberty Arcade on Main Street sells plushes and toys
Lilys Boutique http://www.disneylandparis.ie/shops/disneyland-park/lillys-boutique/
in Liberty Arcade on Main Street has homewares such as candles, picture frames, mugs, plates etc.
Disney Clothiers http://www.disneylandparis.ie/shops/disneyland-park/disney-clothiers/
in Discovery Arcade on Main Street sells general clothing such as tshirts, hoodies etc
New Century Notions Floras Unique Boutique http://www.disneylandparis.ie/shops/disneyland-park/new-century-notions-floras-unique-boutique/
in Discovery Arcade on Main Street sells pins, photo frames, prints etc
Thunder Mesa Mercantile Building http://www.disneylandparis.ie/shops/disneyland-park/thunder-mesa-mercantile-building/
in Frontier Land has clothing and accessories, toys, plushes and houseware themed for Frontier Land / Phantom Manor
Temple Traders Boutique http://www.disneylandparis.ie/shops/disneyland-park/temple-traders-boutique/
in Adventure Land has clothing and accessories, toys and plushes themed for Adventure land
The shop at Pirates of The Caribbean in Adventure Land has Pirates of The Caribbean themed clothes, pins, toys
etc etc, you get the idea.
Basically each themed area has a merchandise shop with themed merchandise. And Mickey and Minnie will be on everything in the themed merchandise shop.
So 1. they need to decide the type of merchandise they want, ie a mug or tshirt for example and 2. decide whether they want a classic Mickey and Minnie or a themed Mickey and Minnie
This is the link to the shops http://www.disneylandparis.ie/shops/disneyland-park/