Hi
I went to Kovalam a couple of years ago, but before I had a baby, so my tips might not be that useful and my memory is a bit rusty, but hope I can help a bit.
The main beach at Kovalam has lots of little guest houses, but not really suitable for families - very basic. We bought a Lonely Planet and found a great place a short rickshaw rise away which had double beds for sun-bathing on, on its own private beach - can't remember the name, but if you look it up in a guide book, you'll spot it. It was run by a European lady, and was gorgeous, quiet, clean and not too expensive.
I thoroughly recommend you book before you go. Buy a book, then check out the reviews on tripadvisor.com. I would also recommend choosing something run by a European - they're just cleaner places.
Kerala is beautiful, but very much geared towards travellers - take lots of anti-bacterial wipes. And if you go on a boat trip on the backwaters - DO NOT STAY ON BOARD FOR THE NIGHT - just do a day trip. V. hot and loads of bugs. And you moore up for the night about 30 minutes from where you get off the next morning.
All the local kids want pens, so buy a load of cheap biros before you go out - the kids shout 'pens' from the banks of the river, and if you throw them some they are just so happy.
It's a beautiful part of India - you'll love it x