MIL won't join but does her own

[LordyPutts]

I am lucky to have grown up near to a steam train line where we regularly saw Thomas the Tank Engine sat on the line.
Now that I have a little boy I want to take him along to this event, but I'm hesitant to pay the extortionate fee because to me I've seen Thomas loads throughout my life, and DS will likely do so too, but ofc I was DS get to ride along and be able to join in with the activities.
I invited MIL and Mum to join us for a family day out, but MIL declined because she's antisocial she says she isn't interested.
She then asks to take DS on a day out the weekend prior, and DH lets slip she wants to take him to see Thomas for the day.
AIBU to not want to let her take him before I can? Especially as she'll pay the eye watering amount when I'm not that fussed to do so.

This. Dont see what your problem is - other than you don’t like your MIL….?

Let her do it gracefully ... you said yourself your not too bothered paying...

Yup.

That is if I am reading the OP right.

OP wants to take DS to see Thomas on a day out. Invited MIL and mum. MIL said no thanks. MIL wants to take DS on the exact same day out the weekend earlier. Without OP or her DH or her mum.

OP is also interested in doing it cheaper, whereas MIL would pay for all the extra, expensive bits. Thus setting up an expectation/ ruining the planned day out.

Getting to board Thomas and just seeing him sitting on the track from a distance are very different experiences. If you never plan to pay for the former, then I don’t see why you would prevent grandma from taking him.

She wants to take him all expenses paid. You aren't fussed. Not really understanding your problem here.

I'm also confused.

All I can make out is that your seething resentment is your mother in law has resulted in a petty jealousy because she is going to take her grandson to see Thomas the Tank engine before you have the opportunity to take him with your mother despite you not actually wanting to take him because you don't like the cost of it?

Put your son first and let him have a lovely day out with his grandmother and you and your mother can do something different the week after that is more in your budget.

Let your son see he has a loving mother and two grandmothers rather than be a role model who is spiteful, jealous and petty.