Microsoft Excel has a large number of possibilities under the Conditional Formatting menu.
- Paste your tune (as alphabetic notes) into a single cell.
- Reduce the width of the cells in your sheet so the result of the next step is not excessively wide.
- Use Data -> Text to Columns to distribute each note into different cells. There are various options for splitting the data by fixed width or by delimiting characters such as spaces.
- Use conditional formatting to colour the cells.
There are two conditional formatting options you could try.
One is colour scales, where the cells can be given for example a rainbow of background colours (I use this at work to prioritise items by colouring by due date.) It auto-normalises so if all notes are present the A would be red and the G would be blue (or vice versa if you prefer) leaving the D in the middle in green, but if you only had the notes A to E, the E (as the last item in the available alphabet) would be blue and the middle note C would now be green.
I can't see an easy way of starting this colour scheme at C, so I hope your kids like the scale of A minor.
The other is custom rules, where you can set all kinds of stuff for each cell (background colour, text colour, bold, underline, etc.) according to its content. If you have a xylophone with an existing colour scheme, you could build a spreadsheet to match it, and paste all your different tunes into it.