Everything will be open - and more e.g. merchandising stalls for T-Shirts etc.
There will be great international atmosphere - some of the fans setup at pavement bars/cafes and make a lot of noise on behalf of their favourite competitor/s. The bars and restaurants are a good vantage point during the marathon if you can get a seat at the outside with a good view. I'd say just keep on the wander along the strip until you see a suitable place.
The bars - e.g. Ruta 66 - are a good place to be later in the night when the last competitors are struggling to finish before the midnight time limit. They usually get a very good cheer !!!
As for the kids I'm not sure. There used to be a kids event the day before on a short course which included the beach.
I'd be very surprised if you were disappointed. I'd recommend you adopt a 'favourite' competitor to support. If you look at the list of competitors which should be available somewhere on here -
http://www.ironmanlanzarote.com/ a week or so before the event, you can maybe find someone you know or from your area or something to give a bit of added interest.
If you are up in time, head for the beach beside Fariones Hotel for the start of the swim at 07:00. You can then head for breakfast afterwards and wait until they arrive back after the Bike leg and start the marathon. I hope you enjoy the occasion - sadly i'll not be there this year.