Is LEGO Junior compatible with regular LEGO? Does LEGO Junior fit with regular LEGO? Say for 8-12 year olds? I want to make sure it is not obsolete and can be used as the child grows alongside the LEGO collection.
Yes. These sets are fully compatible with other LEGO sets.

LEGO Junior is simply a theme within the LEGO System (just like City or Star Wars). It uses exactly the same bricks as regular LEGO. The models in Juniors sets tend to be simpler and use more basic bricks, but the bricks are all compatible.

The main "incompatibility" within the LEGO System is from DUPLO to regular LEGO, but there is even quite a lot of interoperability between these as shown here:

Are DUPLO blocks compatible with LEGO blocks in any way?