We were lucky (well, we deliberately arranged things) that we lived near both our parents who, while they couldn't do regular childcare, could have the kids for a few days. And definitely lucky that both kids were quite easy to look after so we didn't have to feel guilty about leaving them with grandparents.
We went away for our anniversary when oldest was about 2, before DS was born, and went away for a few anniversary weekends when they were small.
We left them to it for a couple of nights (aged 15 and 12) last year because we booked a weekend away for DH's birthday assuming our parents or siblings could have them, only to find both sets of parents and DH's brother (who lives locally) were away for at least some of the 4 days. In the end they stayed 'home alone' on school nights, as previous experience showed both they and my parents found it a bit stressful getting them to school from their house, so they preferred being at home and going from there. My parents were able to have them for the middle two nights, though they were out at a concert one evening. All worked out fine.