You can find the sentencing guidelines for ABH on page 201 of this document (that p.201 of the document, not the .pdf page, which is p.195).
http://www.sentencingcouncil.org.uk/wp-content/uploads/MCSG_web_-_October_2014.pdf
Much will depend on the Magistrates classification of the offence (which they will decide after hearing the full facts). From the brief details you provided it certainly seems as if the offence exhibits “greater harm” (injury caused to a vulnerable victim). It could also be said that greater culpability is evident (weapon – the keys – used). The offence was also committed in a domestic situation (always an aggravating feature) and there were children present, albeit in another room. On the plus side is your previous good character, your remorse and your early guilty plea.
I cannot see this offence being classified as Category 3. It is certainly Cat 2 and possibly even Cat 1. If it remains in Category 2 I would imagine the Magistrates will retain jurisdiction (i.e. not send you to the Crown Court for sentencing) and you could expect either a High Level community order or possibly a custodial sentence which the Bench may feel able to suspend. This is only my guess based on the brief details you provided.
You would be well advised to secure a solicitor to make your case for you. In any event you will also have to pay a Victim Surcharge (£60 if you get a community order, £80 if you receive a suspended prison sentence) plus prosecution costs (usually around £85) and the newly introduced “Criminal Court Charge” which will be £180 for this offence.