The power of the test in the study was calculated by using G*Power 3.1 program. While Type I error is 0.05 and the power of the test is 0.80 (α= 0.05, 1-β= 0.80), minimal sample size was calculated as 62 children (31 children in each group). By considering the losses that may occur from the sample during the study due to any reason, it was planned to include 40 children in both experimental and control groups. Follow-up lost occurred in 5 patients from experimental group and 6 patients in the control group during the study. The study was completed with 69 paediatric patients including 34 in the experimental group and 35 in the control group.

The data of the study were obtained by conducting six interviews with each child and the family. The first interview was conducted before starting to treatment on the first day of the treatment, the second interview was conducted on the 2nd day after the treatment and the following interviews were conducted on the 4th, 8th, 16th, and 21st days and in-mouth mucosa of the patient was evaluated by taking in-mouth pictures following interviewing with the child and the family. Oral care application was conducted four times a day with sodium bicarbonate solution and a lollipop made with black mulberry syrup (after meals and before going to the bed at night). Oral care was first conducted by the researcher, the oral care was then taught to the patient and patient's relatives and the application was evaluated. Figure 1 shows the study design.

It has 16 questions prepared by the researcher utilising literature and including sociodemographic characteristic and oral care habits of the child and family.

Tomlison et al. (2010) developed Children's International Mucositis Evaluation Scale (ChIMES) in order to evaluate the difficulty experienced by children with cancer due to mouth/throat pain developing depending on mucositis. Yavuz et al., (2011) conducted the validity and reliability study of the Turkish version of the scale. The scale consists of six (6) questions and it is an easy-to-apply form that can be answered with the child's facial expressions.

It was prepared in parallel with Oral Mucositis Evaluation Index of the World Health Organization and formed to monitor formation or level of oral mucositis in children on the first, second, fourth, eights, sixteenth and twenty first days.