M&T groups can be quite scary... I've got a friend with a 9 month old and she still hasn't plucked up the courage to attend one.
Have you tried discussing with her how ds could mix with other children, and asking her for solutions to this? Perhaps suggest that she aims for one session a week mixing with other children to start with?
I totally believe it is well within your right to set the schedule. You are absolutely right that you can set the schedule. Perhaps you could find something a bit less informal than a toddler group. Tumble tots is good, and it would keep her busy too, so she wouldn't be left sitting in the corner with no one to talk to, as can sometimes happen to new comers at toddler groups. I have three activities a week that I send my nanny and toddler to. They are not optional.
You could perhaps come up with a variety of choices and let her pick/arrange one. Let's see, suitable for 18 month:
2- Tumble Tots
3- Monkey Music / Jo Jingles / etc.
4- M & T group of her choosing (and your approval of course)
5- Offer her the opportunity to ick something else for him.
My nanny also does a bit of housework (mostly laundry) so I have agreed that on Mondays they stay in and that allows her the time to do this. They go to Jo Jingle on Tues. Swimming on Wed. Tumble Tots on Friday.
Also, there are gymnastics centres around that teach classes similar to Tumble Tots for little ones. For eaxmple, I know Heathrow Gymnastis does a Gymbots class. Where are you? There might be something else nearby?
I wouldn't make her yet. I didn't go to any with my ds, now 7, until he was 3 and he's a happy, popular, sociable boy. And dd is 15 mos and hasn't been yet, mainly because dp was a sahd and didn't want to go but I've found since we swapped and I became a sahm again, I don't want to either! So I won't.
I would go for something more structured like uwila suggested. At least with a music group or gym tots or something you have to talk to the other Mums and also there are not usually so many people there. Perhaps you could get a few leaflets on the different things there are on offer nearby and ask her to choose one?
A agree with you if you said at the start this is what you wanted she should be doing it. Again I understand M&T's can be a bit clicky but I always found Tumble Tots worse for that and the one up man ship is terrible at our local tumble tots. Jo Jangles is quite nice but I felt very silly and none of the other grown ups spoke to me and left immediately.
Is you nanny live in /new to the area? If she has other anny friends maybe playdates instead of organised activities maybe more play areas rather than M&Ts.
As a working mother I felt very excluded at M&T groups and I know that both my very gregarious and delightful Antipodean nannies have felt the same way. Both nannies have eventually found M&T groups they've enjoyed, but they've tended to go with a friend (and her child) so that they don't feel isolated. Otherwise they've gone to very structured classes where there's little room for motherly chitchat or the absence of it.
Reading this back it looks as if I employ two nannies - I should be so lucky! I'm referring to my current and my previous nanny.
I think there's two issues here - your ds going out and about generally, and your ds going to set activities.
Personally, I' would definitely expect anyone caring for any 18 month old of mine to take them out most days, weather permitting. (perhaps the lousy weather has been a reason for lots of stay at home days?) Not big trips, but certainly to the local parks and playgrounds - and not to the same place each day. As well as giving my toddler a change of scene and fresh air, he'd get to see other toddlers and children - all very informal of course. I would feel unhappy if my toddler was not taken out regularly and missed those opportunities for fun and stimulation.
I would be far less worried him being taken to set activites, though, at age 18 months. There's plenty of time to start these later.
I have a baby music "pupil" whose dad came for the first couple of weeks and now sends the nanny. He leaves out the money for her, the place in booked and attendance is compulsary. If you do want her to do something - find something formal, pay in advance and tell her she has to go.
I am wondering though - you say the place is spotless - is she scared she can't keep this up if she is going out?