It's not too complex. A lot of people are commenting based on myths/ what they think should be fair & not the actual law.
As there was no will, the estate HAS to follow the intestacy rules:
No spouse = the estate is split evenly between the children. Therefore, your DD gets 1/3 of the estate and the other 2 children get a third each. Your DD is a minor and therefore cannot agree to a deed of variation involving her share, so this is not an option.
The other 2 children (who I assume are adults) can do what they want with their own share, including giving you money every month to replace maintenance. However, this must come from their share & not DD's.
DD's share must be held in trust until she is 18, when it must be handed over to her in full. You may not touch this money/ use for maintenance. This also must be 1/3 of the estate (assets held in his name - any outstanding debts, liabilities) and not just £30k.
As you will not be receiving any money maintenance or otherwise, your UC will not be affected. If the other family members wish to send you £300 per month from their own share as a gesture of kindness, I'm not sure how UC views regular gifts.
Hope that makes sense, but speak to a solicitor over Mumsnet. A lot of people have no idea what they're talking about!!!!!