The research is clear that sleep health includes regular bedtimes for young children.
That is, bedtimes that remain quite consistent across the week are very important for sleep health because little bodies need consistency. So apart the later nights for special occasions like family functions, it is better for bedtimes to be regular and consistent.
Now, a study form Sweden this year (Ekstedt et al 2017 in Acta Paediatrica) has shown that bedtime is also related to the development of obesity on young children. Later bedtimes, later wake times and the longer it took children to get to sleep increased the risk of a toddler gaining weight. But the most important factor to come out of this study, is that the later bedtimes increased the risk of being overweight.