Hi Safa, Thanks for your question. The sample size calculator by ENA accounts for children under 5years of age. I hope you have adjusted for the percentage of children under 2 years of age. 

If you have adjusted the percentage for under 2 years, for households without children you don't need to replace by other household. You complete any other component of the survey (if there is any) in the household and move to the next selcted household. 

Hope this is helpful

Dear Safa:

Hailu has already provided an excellent answer. I will expand a bit. What you do in a household with no member of a target group, for example a household without an eligible child less than 5 years of age, depends on what data are collected on what target groups. If your survey collects data on ONLY children less than 5 years of age, then there is no reason to collect any data from a household without an eligible child. But, for example, if you collect data on household characteristics, such as water source, sanitation facilities, receipt of food aid, etc., then you must pose these questions in a household without eligible children. If you skip such households, your survey sample will not contain a representative sample of households in which to calculate these household indicators.