The only way my DS would sleep during the day at that age was in the hug a bub.
It's great she'll sleep at night - suggests that she really just needs you close by to fell comfortable right now. It is likely to improve with a bit of time (hopefully). Kate's suggestion is good - have you tried that? Maybe a bit hard with the other kids.
DS had silent reflux and he couldn't sleep lying down day or night until around 10-11 weeks.