I have to say I have heard a lot of support on these anti-witness forums for not helping a Jw be one especially if they are a spouse in the line of 'make it hard for them' and they may come to their senses.
I have often felt the need to say 'Don't you think a person has a right to their personal belief in whatever God they choose?' in the face of people saying' it's witnesses or me?'. look at the comment above me that is implying you shouldn't let them babysit on the grounds that they are witnesses, even though they are your parents.
Is singing praises to your god worship? hmmm if it is the odd religious song amidst just ordinary songs I would say no. But my experience is concerts of the type you are going are worship were everyone goes there to sing and praise the God they worship. And to be honest I not a big fan of religion becoming a social event as apposed to actually being about the God you worship.
Babysitting for someone to goto their worship is a Personal choice but to be honest I was more against your lack of openess over it.
Have you made any restrictions on your parents have you told them they are not allowed to take your kids to meetings while babysitting?
If they did this without telling you on the basis that they don't have to tell you anything, would you be upset?