Via phone by a recruitment agent, who thought I was ideal for a particular role until he heard my toddler, crying In the back ground, he suddenly changed he’s mind and said I was no longer suitable for the job.
The reality is that employers have to tread carefully with staff they employ, protections start from Day 1 of employment, so all the checks and balances are in place nowadays in a way that didn't unfortunately happen before legislation under Equality Act (2010) was passed.
Contractors/Contingent Workers/Zero Hours staff have no such protections where they don't have a contract of employment.
Agency staff which may include CWs are employed by their agency so you could have taken your agency to task, but the reality is if a person is heavily reliant on agency work as their source of employment they probably wouldn't expend all their energy and income dragging them through a Tribunal.
As pp's have said, its best not to give them an excuse to find fault. Yes, of course they should never discriminate, but life is not perfect. A phone interview should be in a quiet room behind closed doors. If your childcare arrangements failed that day, that's unfortunately not their issue, and it opened you to criticism.
They were shortsighted to refuse you future work, their loss. If they needed the post to be filled, maybe they would have 'given you a bye'. Too many ifs and buts and maybes to be absolutely about it.