If you are in Ramsey you could always try May Hill. There is a bridge to get you across during racing so you can watch on both sides and it has a massive park (loads of swings and things) and toilets. Not sure how busy it is during tt but my girls enjoyed 'watching' the Manx there.

As an alternative... On one mgp race day I marshalled at quarry bends and my wife took the kids into the wildlife park for the day.

Have fun!