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You can also consult the official Costa Rican government legal database: Sistema Costarricense de Información Jurídica (SCIJ)

Artículo 1 — The obligation to pay Establishes the employer's obligation to pay an annual Christmas benefit equivalent to one month's salary, calculated according to the law. 

Artículo 3 — Employees who have worked less than a year This is particularly important for household employees who have not completed a full year of employment. It provides for proportional payment when the employee has worked at least one month.
Artículo 4 — The payment deadline The law states: “dentro de los veinte primeros días del mes de diciembre” This means the aguinaldo must be paid within the first 20 days of December.
Artículo 6 — Legal consequences of nonpayment:   The law treats failure to pay as an improper withholding of wages and a serious breach of the employer's obligations. These provisions are available in the full text linked above. 

2. The specific law covering domestic employees 

The law specifically governing household employees is the Costa Rican Labor Code (Código de Trabajo), Articles 101–105, amended by Law No. 8726 of July 2, 2009. 

The official Ministry of Labor document is here: Official Costa Rican Labor Code (PDF)

The Ministry of Labor's domestic-worker guidance confirms that domestic employees are entitled to aguinaldo, whether they work full-time, part-time, by the day, or by the hour. 

3. Official government guidance on calculating the payment:  The Costa Rican Ministry of Labor (MTSS) provides this explanation:  MTSS — Labor rights and aguinaldo information
The official calculation is: Period covered: December 1 of the previous year through November 30 of the current year. Add all ordinary and extraordinary wages earned during that period. Divide the total by 12. Pay the resulting amount by December 20. Importantly, for domestic employees, salary in kind (such as accommodation and meals, when applicable) must also be included in the calculation. 

The Ministry has also published a specific domestic-worker payment guide, including an example of how to calculate aguinaldo: MTSS domestic-worker payment guide (PDF)

This is probably the most useful document for your situation because it specifically addresses household employees rather than employees in general. If you are questioning whether the Christmas payment is 10%, 1/12 of salary, or a full additional month's salary, the law and official guidance establish that it is 1/12 of the applicable earnings, which is generally equivalent to one month's salary for someone who worked the entire year at a consistent rate.