How to tell friend her 3 yr old is too young to be included?

[HIU8]

Please help me with this issue:

DS will turn 7 in November. My friend has a DS his age and they are good friends. My friends younger child is 3. Normally we have invited both kids to both my DS and DD's bday parties. However, I may be planning something that is totally not appropriate for her DD to attend (they come as a foursome--2 parents and 2 kids to all parties). This friend plans only joint parties for her kids (3 years apart) so it's never come up that something may not be appropriate for my DD (who is now 4). How do you handle this type of situation. I have nothing planned yet, but I'm getting worried about this.

[soon2b4]

Could the two DD's have a special playdate? Could you ask the mom to help you out by having your DD play with hers during the party? In my circle, it would not be seen as too forward - you are each taking on one of the other's children.
ETA: maybe even have an idea prepped so you could ask - would you be available to take the girls to do this, since I think DS's party would be too much for them this year?

[HIU8]

That's a good idea. It would have to be something they do not have to drive to though. I do not trust her or her DH driving my kids (their carseats are barely installed and her kids are barely harnessed--as in not always buckled). I can probably come up with something that will work though.