I just calculate it at the end of every month by taking the balance and multiplying by the monthly interest rate. So if you offer 6% per year, multiply by 6/12, or 0.5%. So I have a column called “interest” and the next month’s balance is the interest plus the previous month’s interest, and then I drag both columns down as needed.
To make things easier to see, I also have a “deposits” and “withdrawals” column for each month and calculate that in to the current month’s interest (so current month is previous balance - withdrawals + deposits + previous month interest, and interest is current month * monthly interest rate).
This isn’t how banks do it (they do average daily balance), but it’s good enough and encourages my kids to keep month invested through the end of the month.