Can you extend your fence in some way to block their view of your garden? Very weird that they stare into your garden when your child is there but also if they live on a family estate they are going to have to get used to the noise of kids, especially in the summer.
Go round, talk to them, let them know what you can hear (they may not realise how far their sound travels) & leave it at that. If after that they continue to make you feel uncomfortable then you’ll possibly have to take it further.
Sorry your have poopy heads for neighbours 😔