I think it's important to understand when children are being children, and it's important to understand what is and is not the call for an adult to make. If the kids don't really like grandma because hanging out with her is boring and she seems superficial, that's fine, but if dad says they have to hang out with her anyway that's what they have to do. Yes, because they are children, and yes because she is a relative that their father finds important to spend time with.

I would absolutely agree with you if MIL was actively damaging the kids- manipulating, abusing or scaring them, but their main complaint is that she is boring and takes too many photos. I'm sorry but that is not an acceptable reason to go against their dads wishes and not to visit grandma. You might not have a ton of fun but if it is important to dad it is his right as a father and parent to bring his kids unless they are being harmed. If they are being harmed then it's moms job to step in and put her foot down and pick this battle (AGAINST HER HUSBAND) as one she has. But if the kids are bored and just don't really enjoy it but it's important to her husband - their father, then sorry, but it's just something they have to do.