I'm flying with them today to Bali I'm going with my mum and she has paid the extra to get us seats together.
Last October DH, DS and I flew with them also and didn't pick seats, as DS was only 18months we didn't buy a seat for him, and we did get split up - I was up the front of the plane, and DH somewhere near the back!
I wouldn't put it past them to not plan to seat children with parents, I would give them a ring and ask.

It definitely is a no-frills airline, small and a bit cramped, but doable for short flights and keeping in the budget!