up2late. Can I suggest that you don't hire the bikes in Kinsale? There is a much nicer family activity there and it does not require bikes. This is the walk (already recommended) from Kinsale to Charlesfort and then onwards on foot following a very narrow but safe track right to the tip of the little peninsula. The views from there are far nicer than any Kinsale country roads can offer for you, and a bike would be useless to you on this rough track.
If, however you really do want to hire the bikes in Kinsale, then I recommend the cycle out to Garrettstown, (about 6 miles from kinsale) and then onwards to the Old Head of Kinsale. Garretts is a typical west cork beach area, well loved by Corkonians and it is a big draw for surfers. Continuing on for about another 2-3 miles will take you to the old head of Kinsale, where views of the Atlantic and the cliffs compare favourably with the Cliffs of Moher in County Clare.
You will need to be careful on this cycle as the road is busy, but is certainly doable by cyclists. I recommend that the lead and last cyclist wear a visi vest or similar. This will assist motorists.