Girls or boys? only got experience of girls myself and I love doing their parties, at that age they love fancy dress or themed parties. 6 and 4 are close enough to enjoy same things (our dd's are also 2 yrs apart, now 6&8)
usually we hire village hall and do all the good old traditional games, pass the parcel, pin tail on donkey, lots of balloon based games, simon says, okey cokey, heads, shoulders, knees and toes. the kids love them, honestly.
how many to invite depends on size of class and whether you are combining for both kids, or have cousins etc to invite too. Advise a maximum of 30 tops, we usually have about 18-25, depending how many of the ones you invite can come. Invited whole class in first year of school, more selective now!
Do it, it's the stuff memories are made of, and it will be fun!
We do three parties a year and have always managed to come up with a different idea each time, the easiet by far has to be to hire a hall and a DJ, get someone to recommend one that specialises in kids parties and they will play loads of games and keep the kids well entertained.
In the past we have paid a fortune for a clown, (only to find half the kids were afraid of them) and for an entertainer who was anything but Disco was about half the cost and far more fun.