We love the Amelia Trust Farm! DD is very taken with the "'iggys".
Techniquest is great, if you live in the area it may be more cost efficient to get a family annual pass, I think it's about £40 but will allow you unlimited access.
Margam park is a favourite of ours, we also love the Forest Drive at Cwmcarn (again, £20 for an annual pass is better value if you go a lot), St Fagans and Cosmeston Lakes for walking. DD can spend hours wasting a quid on the penny pushers at Barry Island! Forest drive has quite a lot of child-focused events, we did a great Easter Egg trail on Easter Sunday that was only £1.50, DD loved looking for the 'clues' and pictures of chicks guiding the way, and she won a Creme Egg, little notepad, pencil and eraser that she was well chuffed with! I have the events list in my handbag if there's anything anyone is particularly interested in.
If your kids are happy with a football and a big open space, the Beacons Visitor Centre near Libanus is lovely, nice tea shop and outdoor seating area with stunning views for grown ups, a big grassed area for kids and lots of little birds. We were there this afternoon actually, watching a jet fighter on a training run!
When the weather's nice, Mum and I like to take DD to Southerndown or Ogmore beaches.
Worth knowing that if you have a child who is registered disabled, they will get free access to Jump in Llanishen, just tell them at the reception desk and they will be admitted free. They don't advertise this, I was lucky enough to find out by accident!
Justthe1, we spend quite a bit of time doing the Penarth cliff walk/park and ice cream from Cioni's, you should look out for me. I will be the one looking harried and chasing an errant toddler who is trying to hug a random dog!