I assume the first step you did was a google search to see what was currently out there:
http://lmgtfy.com/?q=don%27t+forget+baby+in+car+device
after you check those links, if you feel your product is still novel, do a quick patent search. Again, google is a good starting point:
https://patents.google.com/
do multiple searches for various key words. When patent searching, you need to think in two ways:
First - am I infringing - Does my product have all the elements described in someone else's independent claim? Focus strictly on claim language for this review
Second - is my device novel - Is there prior art that has been publicly disclosed that would block me from patenting my device myself. This prior art search isn't limited to only US patents, but to publications, brochures, websites, anywhere that is in the public domain.
If you really want to get fancy, go to uspto.gov and look into searching via classifications. This will group all patents based on their 'type of art' and allow you to drill down into specific areas more easily.
https://www.uspto.gov/patents-applic...nd-development
Assuming all that goes well for you (sorry, but based on my knowledge of the industry it probably won't), I'd suggest your next step would be to document your invention and work with a patent attorney to file a provisional application. This will give you the ability to share the idea with others for relatively low cost and keep ownership assuming you find value in the idea and want to continue pursuing the patent. You can choose here if you want to try to sell / license the idea to an established baby products company or continue to develop it yourself.
From there, lots of small product development companies will help you bring your idea to fruition at a cost. A really good one is:
http://www.designthinkstudios.com/
I've worked with these guys often, they have particularly good experience in the juvenile industry.
Good luck.