It is well documented in literature that Heart Rate Variability (HRV) entropies for healthy subjects may present higher values of Sample Entropy (SampEn), a measure of system complexity, when compared with subjects with specific pathological conditions. Nevertheless, for new groups and conditions, nonlinear measures must be considered and discussed. This work proposes the use of SampEn to analyze 2 different groups of children during 24-hour ECG/HRV monitoring. The considered database consists of 20 children (age 9.18 ± 0.68), divided in two groups: 10 Normal children (5 female) and 10 children (5 female) with medical history of intra-uterine growth restricted (IUGR). Four different entropy calculations were considered: the complete 24 hours trace; before sleep time; during sleep and after sleep time. Statistic t-test was considered for intra and intergroup comparison. The results showed significant difference (P < 0.0349) during sleep time interval between normal group (SampEn=1.2325±0.094) and IUGR group (SampEn=1.3517±0.122). This result may indicate that IUGR children may develop higher levels of complexity during sleep time. The other intervals presented no significant difference in intergroup comparison. An intragroup analysis was also considered and both groups showed significant difference between before sleep and during sleep intervals. For the normal group (P<0.001), before sleep (Sampen=1.0237±0.103) and during sleep (SampEn=1.2325±0.094). For the IUGR group (P<0.013), before sleep (Sampen=1.1173±0.223) and during sleep (SampEn=1.3517±0.122). The results indicate that SampEn may be considered as a measure of HRV complexity to monitor normal and children with medical history of IUGR. Further studies with other nonlinear measures and with an expanded database are going to be considered.