Good Luck - prepare as many suggestions as you can and make it clear to your boss that you are willing to work hard to make this work - if poss I would send your ideas before the meeting. This could mean letting them know that you will send something over the weekend if that is possible.
Firstly, I would open up the discussion regarding there being no opportunity to use the money saved by you moving from a full time position to a three day position. If you go to three days, they will make a saving, while there may not be room to make a permament additional head, it would seem silly to simply loose this budget. Temporary part time staff may fall outside of the definition of a "head".
This could be used creatively - some ideas:
Additional admin support across the Account Management team to free up resource to cover your customer 2 days per week,
Additional training and / or pay rise for other staff to enable them to take decisions in your absence,
Temporary support to create robust standardised policies and procedures, working practices, documentation across all customers so that any Account Manager could step in on any customer account to cover holidays, absence, assist during times of peak activity. Making everyones life more efficient / easier could allow you to get what you want.
Flexibility: Not knowing your childcare arrangement, you are in the best position to discuss this but could you be creative here - if you were to work two days in the office and say three hours per day from home on the other days, could you find a child minder who has mostly school age children to look after your child (if pre school) over lunch and nap time or if you have school age children, could you do this during school hours? Some nannies are free during the day if they have school age charges too.
Managing cover for customer and escalations during your absence - how is this managed during holidays? What happens if you are in an all day meeting? Is the work load equally shared across customers? Are some customers more complex / needy? Could this be reviewed to see if there is any over resourced account / under challenged account managers / junior staff with ambition? Given a robust escallation system, could junior staff resolve low level issues and if required, contact you on your day off to see if issue needs to be escalated to a higher level or could wait till your return. You could do Monday, Wednesday, Friday, check high priority issues twice on your day off (and get paid for it) say at nap time (or half hour of telly) and again at bed time or use more formal childcare arangements. You could also offer to work additional days (with pay) if given enough notice, in peak times of go-lives, upgrades etc.
Extra work: Think about your full time job, what the tasks are and how much time they take up. Are they all necessary, could some be made easier / quicker (sometimes spending time to fix a process can save significant amounts of time on an ongoing basis)? Can some tasks be performed by other departments - do you chase invoices, get involved with technical issues etc? Would admin support help by managing your diary, resolving some simple queries, preparing for meetings, writing reports? Could project work be allocated elsewhere? Also, think about the structure of your team / larger organisation - is there any scope to change your role within the context of other roles?
Lots of work in preparing the answers, but worth it if you get your life work balance!!