For this particular project the consistency of the paint is the most important. It must be a soft paste that sticks to the hand and to the canvas. If the paint is too thick, just add a drop of water at a time. If the paint is too liquid, you'll end up with splashes and incomplete handprints.
With children that age you can use almost any type of paint, like:
- Acryllic paint from a craft store
- Watercolors in tubs (the ones intended for primary school students)
- Wall paint
- Finger paints inteded for very young children (not recommended because of low quality)
In the linked pictures they used semi transparent paint for the handprints which beautifully highlights all the lines and details of a hand. Your best bet to get such a semi transparent paint is an actual art store or a well-equiped craft store. All of the big paint manufacturers have a code for the properties of their paints. This is just an example from Schmiencke:
Schmienke College Acryllic Colourchart
Semi transparent paints are marked with either a white square with a diagonal line or a square with one white and one black part. Other manufacturers may use a different code, so look for the color chart of the respective manufacturer. Some paints in the linked chart like orange, bordeaux, ultramarine blue, may green and ocre highlight differences in thickness of paint very nicely.
Paints that are not suited are:
- Watercolors in hard blocks to be dissolved with water (you'll have a hard time getting them to the right consistency)
- Oil paints because they need extrelely long to dry and you need chemicals to clean them that are harsh to the skin and airways
A word concerning the execution of the project:
The handprints in the linked picture are too perfect. Natural handprints almost always have a gap in the middle of the palm because that's where the hand curves inwards. If you put your hand flat on a surface, the center of the palm won't actually touch the surface.
This part of the palm will not touch the canvas unless you gently press the canvas from behind. Since the canvas is suspended from the top of the wooden frame, you shouldn't let your children press too hard on it anyways. Either support the canvas (not the frame, but the fabric) from the backside or work together so that one child presses their paint covered hand on the front and you press the same area from behind.