Making something for DD out of MDF - how do I paint/spray it?

[puzzle]

I have bought some 3mm thick MDF board that I want to paint to make sandpaper letters for my DD. But I don't know what sort of paint to use.
It obviously has to be child safe, and quick, easy and cheap to do.
Can anyone help please?

You will need MDF primer first then you can add paint you want (emulsion, gloss, whatever).

Thanks.
I was hoping it could be done with just one paint.
Now I am worried it will be difficult to find child friendly paint.