We first took our daughter to Greece when she was 4 months old, and have been going once or twice a year since then, including a few trips to Plakias. She is now 13. We would usually order a few things and share, which is perfectly normal in Greece. Or sometimes she would have a starter for herself. As she got a bit older she turned to spaghetti, omelettes, small fish or souvlaki. You are best with freshly cooked food, as meat that has been kept warm for several hours isn't always safe for small children. Several tavernas in Plakias have pasta, pizza and "bifteki" (very nice hamburgers), and are generally very friendly towards children.
As for play facilities, we would eat at a taverna on the beach side and let her loose on the beach to play with the other children, usually Greek. But perhaps your children are a bit young for that at the moment. We always took a colouring book and pens, and she would often entertain herself drawing on the paper table-cloths, which are disposed of anyway, so not an act of vandalism!
Efigenia, as mentioned above, isn't far from Plakias Suites, just up the road beside the new development, turn right at the top (on to the main road) and a left into the next road, where it is a short way down. The way would be very dark at night, but it is often open in the afternoon. It does meat as well (at least when I went), but the vegetarian food is lovely.
You have made a good choice with Plakias - enjoy it!
Arnaki