Some tips on starting this project:

1. Talk with your child about the significance of what they are painting.  Whether it is the du'a, or Allah's name, explain what it says and ask them what they think about it.  Depending on your child's age, you can even write down some points to remember later!
2. Set up their painting area with a cup of water, some brushes, napkins, and the paints.  You may even want to play some nasheed or Quran as they're working to make it even more special!  Older children can try to say the du'a as they paint it as well.
3. Use acrylic paints or paint pens with a variety of brush sizes (thinner brushes work best for small areas)
4. Encourage your child to pick colors that they like best, and to also think of additional patterns or designs they would want to include. 
5. It may be easier to paint the background first and then work on the smaller foreground designs and letters.  You can even go over the entire plaque with a light (diluted) color to give it an even coat of paint before detailing.
6. Allow your child to take their time with the painting and enjoy the process, you may even be inspired to paint one too!  It could be a great project to do together as a family.
7. HAVE FUN!!