Unblock air brick or fit trickle vents

[Crayon]

Like a lot of people in the UK we get bad condensation during winter upstairs - around the window frames and it turns mouldy in the corners which isn't going to be doing our 2 little ones much good. I want to solve this once and for all, so my first step is to either fit trickle vents or unblock the air bricks (which were plaster boarded over by the previous owners when the house was redecorated). Our windows are double glazed and the frames are UPVC (dark wood effect) and are quite old. My question is what would you do first, unblock the air bricks or fit trickle vents. I'm guessing that unblocking would mean drilling from the outside to find the 4 corners of the vent area inside, cutting it out of the plasterboard and fitting a grill. I'm concerned that there may be a strong draught with the air brick vent for my 2 year old, does anyone find this with air bricks and are there more sophisticated grills that can be bought that are less draughty? Trickle vents seem quite drastic and invasive for the window frame and look a bit of a mess but I get the impression they are less draughty?

Thanks in advance for any advice/experience you can share on this.