The weather can be unpredictable in mid-September.

Weather history for 15th-29th Sept for the past 10 years:

Burgas region

Varna region

Can't help on hotels or childrens' entertainment.

Baldur

Hello Celine,

Myself and girlfriend are just back from sunnybeach. We stayed in the "Iberostar sunny beach resort".
This is one of more expensive places to stay but we loved it. We dont have kids but this hotel is ideal for anybody going to sunnybeach with children. From 10 till late theres the STAR FRIENDS team who
do their best to keep both kids and adults entertained. Fun and games for the kids like darts, water polo, racing in the pool on blow up animals, dancing and a show everynight at 930. Also for the younger kids theres a kids club with skittles, a small play ground and trips to the beach with the kids so you cn sit back and top up your tan.Theres also shooting, darts, waterpolo,bingo everyday for the older generation. And i'd highly recommend the food there, we went halfboard and did not hear one complaint from any guest for the whole week. The variety of food each night is very immpressive.

Anyway hope this helps.

Hi - personally I wouldn't bother to go AI. It's pretty cheap to eat in the resort and generally reviews of hotel food are not good. Except at the Iberostar of course

Hi Celine. Just a couple of comments - first off, how old are the kids?, it might amake a difference to what people recommend; secondly, the middle of September s getting towards the end of the season in Bulgaria, and you may find some places winding down - earlier (even by a week) would be better.
Lastly, the AI thing. I agree that, price wise, you won't save much by going AI - we certainly don't . That said, we find it is very convenient not to have to carry cash down to the pool, etc., and also not to have to drag the kids off to lunch when they're not hungry/don't want to leave the pool.
If you can give some idea of the ages & interests of the kids (and you!) then we can suggest resorts & hotels that might suit