The Organic supermarket has probiotics especially for children. The amount of probiotics in commercial yogurts and actimel is very low in comparison with the higher quality supplements. I think Biocare range has a nicely flavoured powdered version which can be stirred into juices, cereals etc.
It says 3 years old and over here
[i'>Actimel is suitable for everyone from age 3 upwards when consumed as part of a healthy balanced diet and lifestyle. It is not formulated to meet the specific nutritional requirements of an infant or young child. If you have any concerns about consuming Actimel we recommend that you talk to your GP[/i'>
http://www.actimel.co.uk/About/FAQs.aspx