It was along time ago (20yrs but I think they'll still be looking for the same things). I don't think it's a problem with regards to taking your approach regarding being happy,friendly and building a rapport. That's what you'll need to be able to do day in day out.
It's likely one of the the role plays will be an 'angry man' scenario where you'll have to be able to take control of the situation and calm it down. Other role plays will probably just expect you to use some common sense and you need to remember to try and elicit as much information form the actor as possilbe.
With regards to the competancy based interview I assume you have a copy of the competancies that are being tested. If not these are on the West mids site
EFFECTIVE COMMUNICATION
Communicates all needs, instructions and decisions clearly. Adapts the style of communication to meet the needs of the audience.
COMMUNITY AND CUSTOMER FOCUS
Sees things from the customer?s point of the view and encourages others to do the same. Builds a good understanding and relationship with the community that is served.
PROBLEM SOLVING
Gathers information from a range of sources to understand situations, making sure it is reliable and accurate. Identifies risks and considers alternative courses of action to make good decisions.
RESPECT FOR RACE AND DIVERSITY
Understands other people?s views and takes them into account. Treats people with dignity and respect at all times no matter what their background, status, circumstances or appearance.
TEAMWORKING
Works effectively as a team member and helps build relationships within it.
You need to think of a couple of examples to cover each but they are looking at you having common sense and some confidence (although they accept you will be nervous).
Good luck!!
PS your kids are gorgeous