It’s whether asking DP to help with DD is too big an ask?
For 1 week - not at all.
For 2 weeks - difficult if he is not in favour.
More than that, unreasonable.
Without knowing more about how your DH’s work patterns are, whether the school is close by etc, it’s hard to say.
I would expect my DH to negotiate with his employer reduced hours or some parental leave for up to 2 weeks, but not more. But my DH can WFH almost full-time and already shares some school run & childcare duties as I work too, so we are set up for this.
There’s also a difference between whether he’s working till 9 at night because his job is insecure or whether those are his core hours (if he’s working internationally, for instance l, so starting later) or whether his employer has just got used to shafting him and he’s a workaholic who’s never had to step up before etc.
Only you know.
But in principle - your DM should pay for a bit of home help, you should go for 1 week or perhaps 2-3 days per week, around your DH’s in-office schedule if possible, and your DB needs to help too.