Looking at the comparisons on the HIA website, it seems that the children prices are €200 less (each) than the most expensive child rate. I'm not sure if there's a child rate offer on with Aviva, but they're only charging €95 per child.
Benefit wise, they seem broadly similar. The out-patient cover on Aviva is minimal, but (a) that suits you, and (b), the other products have higher excesses anyway.
Here's a link to the three side by side - [broken link removed]