You could try Mix in IFC (currently closed for renovations, but apparently opening soon). They have an outdoor area with tables.
There is also Triple O/Whitespot in Exchange Square, just near IFC that has outside tables and your toddler can run around Exchange Square. My daughter loves their milkshakes.
With anything in Central, you are best to get there early. I'd aim for an early lunch around 12noon. They fill up really quickly.