There is a pre-school at Bryn Deri but can't coment on it as my 2 never went there. There is also Park Road Nursery which is a private pre-school and does mornings only or a new day nursery in Morganstown. We only moved to Cardiff 2 years ago and DS went to a day nursery near where I work. DS2 did go to Park Road on a friday morning for a couple of weeks but he didn't like it. Not sure if that was because he only went one day a week or because it was so different to his other nursery - in Park Road I never heard a child speak which is odd for kids age 2.5 - 4!!
In terms of activities, there are lots of things that go on although you may struggle for your three year old. If you visit the Radyr & Morganstown website, you can have a look at the old churchrooms diary as they have dance angels etc. There is also a drama school at the Comp on a saturday morning. DS goes there and is in the 4 - 7 age group. He really enjoys it and they are working towards a performance of Aladdin next month. The comp also have several swim schools that run classes in the school pool.
There is a scout hall and a guides hall. The waiting list for Beavers though was closed when I tried for DS1 so you may want to put your DS name down straight away!! Couldn't even get DS on the waiting list!! There is Radyr Rangers for football, although I'm not sure if they have to be 4 for that. Radyr also has a cricket club & tennis club. The tennis club does do half hour lessons for 3 year olds and they are only £2.50 a week. You can also join the tennis club as a social member if you don't want to play (otherwise nearest bar is in Morganstown!)
With regards to Summer clubs, Superstars will be runing for 3 weeks in the summer (I think its the first 2 & 4th). DS will be going to ActionPak which is in Whitchurch as they do longer hours & is more structured in terms of activities. They even take them swimming once a week! It is really well organised & is run by Acorn Nurseries.
If your 5 year old is going to Radyr Primary and you need breakfast or afterschool clubs then Latchkey are very good. They pick up and drop off the kids and are based at the Scout Hall on Heol Isaf.
I'm not 'up' on the girls activities as I have 2 DS but I know some of DS1's friends in Y1 do go to Rainbows and ballet.
Will stop now - you probably didn't expect as essay as a response! Any questions, please ask - I know how difficult it can be moving to a new area with 2 DC!!
Welcome to Radyr - it really is a lovely place to live!!