No, you won't be holding your children back if you have a nanny - though you will need to insure that you get the right nanny, as you won't want a nanny who stays at home all day - you will want a nanny who goes out and about, meeting up with other carers who have children of similar ages. As a nanny, I meet up with some local childminders - some of the children have now know each other for over 2 years. We go walking as a group regularly - the 3 & 4 year olds love this, especially this time of year when they wear they wellies and get some freedom to walk ahead (it's a route they know very well, not near any roads) and they play games and chat amongst themselves as they walk.
Not sure what you mean by playscheme - I'm in England and to me a playscheme is a group which runs during school holidays, providing care for school aged children, where the children have fun playing and doing arts/crafts.
For younger children (2.5-5yrs) we have pre-schools (formally called playgroups) which are 'learning through play' and run for a couple of hours, often in the morning.
I nannied for a family where their 3 year old went to pre-school 3 mornings per week. This worked well, as he got some time to be away from me and thus to become more independent. I also got a couple of hours to sort the house out.
So a combination of nanny and pre-school could work well... though having just a nanny also works well. Pre-school sessions in England are subsidised from the term following a child's 3rd birthday, thus why some parents will choose to send their child to pre-school for a few hours a week. It should be noted that during these hours, the nanny is still being paid - as the nanny is the emergency contact, plus will be able to get on with some household duties at home, or be caring for younger children.