majorstress, just be firm and do what suits you and your dds. One time it might ok for guests to come in and play, another time it won't. That's life and you are totally within your rights to chop and change. Say no with a smile on your face. Don't offer long reasons to these children. Just say it's not convenient today but might be tomorrow.
I have had about 5 years of coping with children playing out and inpromptu visits. Sometimes it's great to have a houseful, most times it's not.
Things I have learned: never let your children play with small but valuable toys in the street - dolls house furniture, brio train tracks etc. Never let your child take out anything that's too 'wantable' by older children, such as gameboy games, swapping cards or footballs - sadly, some children (mostly older ones IME) are light fingered - or play with something till it breaks.
The same goes for bikes and other wheeled things - make sure your child or children of the same size, ride them - not bigger children who can break them.
Never let children you don't know go into the bedrooms. Make upstairs a totally no go area - this is partly for your safetly as the adult in charge - if a child said to their parents, I was in 'so and so's bedroom and *** happened' this could put you in a worrying position.
And if a young child wants to come in, say 'no, not unless you ask your parents first - I cannot let you come in without their permission' In your case, majorstress, this could work if the parents of these two girls are asleep during the day. They can't give permission so no invite in. If you want, you could also ask for a phone number and say you will ring the parents to check if it is ok.
I think it is very important for your children to know that it is unacceptable and dangerous for any young child to go inside any house without their parents knowing. If they see you allowing visits behind parents backs, they might think it's ok to do this themselves, which is a safety risk.
If the children want to come in, it may be better to let them sit on the doorstep or play in the hall - I don't know if this would work for you, but I find this a good compromise. I then being out a tray of drinks for them, but the gang know they cannot come inside.
Right now my sons are having a water fight in the street and I have said no to having guests inside - both my sons have asked and also one of the neighbouring boys has asked me to my face. I've just said, it's lovely weather out there so go and enjoy it!