We have a set like this Small World Knight Playset and it has been great fun. It says suitable from age 3 and that's probably about right. The armor is still a bit big for my 3.5 year old son (not that that stops him!)
The sword and shield get the most use. The helmet tends to fall off, I could probably attach some velcro but the kids seem to manage.
We've trained the kids to use the sword safely. Yes, it's plastic, but it can still hurt if someone gets hit with it. You can make the training fun and tell your child they're learning how to be a knight! The rule in our house is that the only thing you can hit with the sword is the shield.
So you can get someone else (usually Mom or Dad) to hold the shield and then hit it. But not too hard - when you're training, you're learning to be accurate with the sword. It's actually pretty good for motor control.
Did you like the Knight Costumes?
Is the Mike the Knight costume available yet?
I could see my grandsons in these costumes. They'd love it.
My daughter would love these - she loves Mike the Knight and we enjoyed making her a cardboard sword and shield not so long back. I'd love to get her a proper dressing up set for her birthday. There are some excellent books on learning about knights and castles as well.