When I work with children, these are a few tips I like to share. I also find that there are times I need these same reminders myself:
1. Everyone is creative - it just takes practice...like riding a bike.
2. To keep colours vibrant & colourful, don't mix all three Primary (red, yellow, blue) or Secondary colours (orange, green, purple) together.
3. To keep things interesting, use some Warm (colours like the sun - orange, pink, red) and some Cool colours (skies/grass/water - blue, green, purple).
4. When an image does't 'feel' right to you, try looking at it from a different perspective - step back to view it from a distance, turn your drawing or painting upside down, or look at it in a mirror.
5. Use thin layers of colour & let each layer dry in between because mixing all the colours together will make brown.