Hello Paul Aileen and wee Euan,
I'm sure you will be excited already at the thought of your first holiday as a family.
Arillas is ideal for families, especially youngsters. The beach lives up to it's Blue Flag status and is excellant for the wee boy, with water sports available for mum and dad. For an alternative day you can hire a motorboat with driver who will take you on a tour around the bays.
A day away from the beach (not far though as it faces it!) is the Mirage, take a look at their website here
http://www.mirage-arillas.com/ It has a large pool, a childrens play area with swings etc, a pool table for the older ones, bar and restaurant.
We have stayed in Annas Studios and the studios are very spacious and clean. Anna, Costas and family will make you most welcome. You will find many posts acclaiming the studios if you look back on the forum pages. One little thing I feel needs taking into consideration and is only fair to point out, is that the studios are up a steepish incline. Not too bad in the cool of the evening, but in the heat of the day it will be a different story when pushing a buggy and all your beach paraphernalia!
Our grandson is 2 and a half years old and he loves it in Arillas, we have not long returned. Everyone is so tolerant and kind with children.
If you don't want to walk far but would like a change of scenery take a walk along the headland to San Stephanos where there is a lovely sandy beach too. I can tell you from experience that the path from Arillas beach to the headland is steep, but we are a good bit older than you and we managed pushing a buggy.
'hope this helps and please ask if there is anything else we can help you with.
Eileenx