The key is that as you design your phased hours plan with either your manager and/or Occ Health, that you are all clear of the goal which is to achieve your contracted hours by a set date. It can be tricky if you start to depend on the phased hours for childcare. You and your GP/Occ Health will need to distinguish when you are fit for your contracted hours as oppossed to when you have your childcare sorted.
Other tips would be : expect emotional good and bad weeks, and for even appreciated colleagues to irritate you! First week is exhausting (catching up with everyone), second week a bit more settled and enjoyable, third week less of a novelty and the tiredness kicks in so you can feel lower than usual. Try to keep that in mind and not feel disheartened if it happens, it is all part of rebuilding your emotional and physical work stamina.
Ask for a contingency plan i.e. if you have a real low day and just can't face going in, is it possible to have a prearranged system so that you can work that days hours back later in the week? If it isn't working and you aren't coping, talk to your manager/Occ Health/GP sooner rather than later, better to regroup and re try a bit later than sink low, and start using up holidays days to avoid work.
If at all possible, keep an hour or so in the week just for you - not work, or home, or parenting, and do something that relaxes you.
Hope it works out well for you, good luck.