Fabric dolls are fun and easy to make, but they're made to be used, and so their durability must be taken into consideration. There are sprays that are available that will seal the fabric, but they will also change the feel and texture -- giving it a hard "lacquer" feel as opposed to feeling like a soft doll. There are other methods, however, of making sure a doll's face stays put for as long as possible.
Liquid Laminate and other, similar, products will seal fabric against everything, including water or air. This results in making the fabric hard and shiny -- that's what products like this are supposed to do, but this mars the desired finish. It's possible that watering down products like this will reduce the hardening effect, but it would also remove some or all of the sealing properties of the product.
Scotchgard is a spray that makes fabrics of all kinds water resistant. It will prevent stains and keep water from getting into the doll, which will keep any fabric paint or marker on the doll better. However, Scotchgard leeches out of fabric over time and is not safe for human consumption. A child playing with the doll may get it on her hands, or worse, put the doll in her mouth and end up swallowing Scotchgard, which could cause illness, especially in a child sensitive to chemicals.
Fabric Paint Used Properly
Fabric paint, used according to instructions, should be long wearing on a doll's face. Heat (such as an iron) can be used with some brands to set fabric paint better in the fabric, but fabric paint is supposed to seal itself to fabric. Most fabric paint, pens and markers will last as long as the fabric lasts. If it fades later due to sunlight or usage, the paint can be touched up.
Sewing a Face
An alternative to sealing paint onto a face would be to use thread and embroider a face onto the doll. The right thread should be tough and durable, and stand up to hard use, without introducing extra chemicals or changing the way the piece feels (through the use of a lacquer-type fabric sealant). There are many patterns for doll faces available online, and doll's faces tend to be simple, which should make this a fairly easy project.