Hi baiyu,
I've lived in Cheltenham for around 5 years now, and have a 2.1 year old ds; and we rented when we first moved here. We rented through CJ Hole, and they were good, Isherwoods is another big letting agency round here. And most of the estate agencies handle lettings too.
There are lots of toddler groups and classes, it just depends on the area you are living in. Areas - well, it depends what you want! Can't list them all, but here are a few of my favourite areas:
Charlton Kings, southwest - quite expensive, out of the town, lots of families because it has great schools, quite a lot of detached houses.
Leckhampton, south - lots of Victorian terraces, great mini high street, fairly expensive again.
Tivoli, more central - some really nice terraces, again higher than average price, quite popular for renting
Prestbury, northeast - out of town, got a 'local community' feel to it away from the town centre, good mix of houses, but not brilliant links to the south or west if commuting
Pittville - just north of centre but close to town, great parks, some lovely architecture - we rented here, but didn't buy - not ideal for our commute and the schools in the north are not as good as the south, unless you're closer to Bishops Cleeve.
Hope that helps for starters - happy to try to answer any specific questions!