AIBU or is DP

[shemustbetiredofsomething]

I'm self employed. DP has a full time salaried job, 3x my income. We split bills and household expenses 50/50.

DP is off work for a week as their family are here staying with us. I have a 9 year old son from a previous relationship. DP offered to take 9 year old out with the family (on DPs side) tomorrow as I have a crucial work meeting that I can't miss about a time-sensitive project, and I need to commute for it. They have planned a nice day out together, all seemed fine.

DP said this evening that DP is going to go for a run tomorrow morning and leave my DS with DPs 14 year old niece 'for an hour or so'. Niece has never looked after DS before - and he can be quite challenging. She's also quite immature for her age, barely communicates with us, and I'm not confident she would know how to handle an emergency.

DP thinks I'm being unreasonable. I'm not a helicopter parent by any means, but if anything were to happen, I would be 40 miles away and DP would be around 10 miles away, and niece doesn't know ANYONE in our area. DP said I should just go into work later instead (???!!!!!) rather than DP not go for a run, I'm supposed to dick my work about? As freelance/SE I can't afford to mess up opportunities, and this is a big one. I'm infuriated but also quite sad that 'a run' takes priority over my DS and my work. DP has been running for about a year, I've been at every finish line, given up my own weekend plans countless times to support, and just this once I'm putting my foot down. Or trying to.

WWYD in my circumstances? AIBU?

A 14yr old supervising a 9 yr old for one hour sounds fine to me. But if this isn't acceptable for you and DP then of course it should be your DP who gives up his run, not you going into work later. That's very silly.

It is completely irrelevant if some users would leave their 9 yr old with 14 yr old, he said he would look after him. Obviously the money split is separate but also shows his character and lack of care for you

He's failing you massively if his asinine excessive exercise time means breaking a promise to you and it doesn't matter if Mary Poppjns herself was babysitting. You need to tell your so called partner to jog on and YOU run to a divorce solicitor ASAP

run to a divorce solicitor ASAP

they aren't married.

50/50 finances might be unfair but depends what 'household expenses' covers - I notice you don't mention rent/mortgage. Is this DP's house? Are 'they' charging you rent? If not, I think 50/50 seems fair.