Sidelined for going part time? Ask the expert

 My first question is who told you that as a part-timer you couldn't apply for a director's role; and what role does he / she fill in your company? 
 
If the person doesn't belong to your company, then take very little notice of the advice.  You'll realise I'm saying ignore your friends' advice, too, because they don't know enough about your value as an employee nor about your company's employment practices.  If the person offering advice isn't in charge of your UK company's policy and / or recruitment to Director level posts, then the opinion they've given may not reflect that of the Board of Directors.  Similarly, if it's an opinion offered by an executive from the American parent, that person may not be aware of the UK's cultural and legal differences from the States.  I'm no lawyer, but I think as a parent with very young children, you're entitled to ask your company to offer you part-time work in posts at any level (including that of Managing Director!) and there's more than enough case law to discourage UK-based HR Directors from putting legally unjustifiable obstacles in the way of mums' promotion opportunities.
 
Your best bet, though, is to think like your company MD and work out how you'd be able to fill the Director's role to perfection even though you work part-time.  Ask yourself why might the company expect to benefit from having full-time rather than  part-time Directors?  How real is the perceived advantage of having a full-time Director?  Would having two part-timers in the role be even more advantageous to them?  Prepare your case well and then have a discussion with your company's top management concerning your ambitions to be a Director and the particular benefits the company would enjoy from recruiting you (and partner if necessary) into the role.              
 
I think you'll become resentful and disengaged if you don't take any action at this stage to progress your career further.  You may harm your longterm career prospects if you allow yourself to be left behind in your twenties and thirties.  Also, there is potentially much to be learnt career-wise from working at Director level in a company newly taken over by an overseas parent. 
 
GOOD LUCK! 

Linda Whittern is Director of Careers Partnership (UK), a careers consultancy for women and men at all stages of their careers.